The only place where I see taking it really making sense is something like Kusari-game wielding monk needing to take it to get proficiency in whip so his Kusari can be recast as a whip with a sickle on the end. Yes a wizard could use it to walk around wearing a rapier or long sword and actually be proficient with it but WHY?  With the loss of exotic weapons and the nonexistence of weapon expertise what is the reasoning behind this feat?

I think this is strictly just in the game because it was in past editions, except in past editions it may have been useful.

In past editions because of the existence of exotic weapons it was useful. Now with slight variations on the two examples I gave  it is not. In my campaign I simply ruled that a chain was a simple weapon. And a long chain was a simple weapon with reach. I’ve worked with chains as weapons including double weighted kusari and, like nunchaku, they take some time to gain proficiency but are not spectacularly difficult  as long as you build your speed practice slowly so your body does the motions correctly automatically before you do anything fast.

I mainly see it as useful as a feat a DM could grant through down-time/RP activities. Like if your cleric has been training in the use of a Warhammer in their down-time, they could eventually earn the Weapon Master feat to give them proficiency in it. I mean you could say given a DM could tweak things to grant proficiency with a particular weapon regardless of the feat, but that's true for quite a number of feats. I welcome feats like it because much like things like Keen Mind they are feats people will generally pick for their RP/Flavour value rather than just trying to min-max the hell out of their characters (especially if they can earn it in downtime etc rather than using an ASI).

There are classes that have a limited list of weapons they can use, and they can't use anything else proficiently.  This lets you add weapons to that list.  It may not be all that useful to a caster type, but it doesn't do any harm and could possibly prove to be useful.

The is a wonderful thing for a Rogue.  They can use simple weapons, hand crossbows, longswords, rapiers, and shortswords.  Add any 4 more weapons from the Martial list.  How much would a Rogue like to be able to use a Greataxe for Sneak Attacks?  Would a Rogue find a Whip useful?  How about a Halbred, for a lovely melee ranged Sneak Attack?  Maybe they could use a Maul to really hammer things out.  Maybe they would like to do something at long range?  How about a Heavy Crossbow?  I'm sure that with Sniper, a d10 ranged attack at 400 feet with Sneak Attack would be handy.

So say the stars have all aligned for a Variant Human 4th level Assassin with a 20 dex and the Sniper and Skulker feats.  They have the Initiative, they are undetected, so from 400 feet away, 2d10 +5, 4d6 +5 sneak attack.  That's an average of 41 points of damage.  With a Hand Crossbow, from 120 feet away they get 2d6 +5, 4d6 +5 sneak attack an average of 31points of damage.  It's only 10 more points of damage, but they don't give away their position when firing, so they can keep doing that until someone somehow manages to beat their average of a 19 DC Stealth score from 400 feet away with a perception test and no idea where to look.

Quote from Geann >>

There are classes that have a limited list of weapons they can use, and they can't use anything else proficiently.  This lets you add weapons to that list.  It may not be all that useful to a caster type, but it doesn't do any harm and could possibly prove to be useful.

The is a wonderful thing for a Rogue.  They can use simple weapons, hand crossbows, longswords, rapiers, and shortswords.  Add any 4 more weapons from the Martial list.  How much would a Rogue like to be able to use a Greataxe for Sneak Attacks?  Would a Rogue find a Whip useful?  How about a Halbred, for a lovely melee ranged Sneak Attack?  Maybe they could use a Maul to really hammer things out.  Maybe they would like to do something at long range?  How about a Heavy Crossbow?  I'm sure that with Sniper, a d10 ranged attack at 400 feet with Sneak Attack would be handy.

So say the stars have all aligned for a Variant Human 4th level Assassin with a 20 dex and the Sniper and Skulker feats.  They have the Initiative, they are undetected, so from 400 feet away, 2d10 +5, 4d6 +5 sneak attack.  That's an average of 41 points of damage.  With a Hand Crossbow, from 120 feet away they get 2d6 +5, 4d6 +5 sneak attack an average of 31points of damage.  It's only 10 more points of damage, but they don't give away their position when firing, so they can keep doing that until someone somehow manages to beat their average of a 19 DC Stealth score from 400 feet away with a perception test and no idea where to look.

 Isn't sneak attack limited to finesse weapons?

Quote from scatterbraind >>

I think this is strictly just in the game because it was in past editions, except in past editions it may have been useful.

 It really wasn't.  Under 3rd Edition rules, there were very few exotic weapons that were worth spending a feat on- in most cases you were better off sticking with a martial weapon and taking a feat to improve your usage of it, like Power Attack or Weapon Focus.

Quote from Geann >>

There are classes that have a limited list of weapons they can use, and they can't use anything else proficiently.  This lets you add weapons to that list.  It may not be all that useful to a caster type, but it doesn't do any harm and could possibly prove to be useful.

The is a wonderful thing for a Rogue.  They can use simple weapons, hand crossbows, longswords, rapiers, and shortswords.  Add any 4 more weapons from the Martial list.  How much would a Rogue like to be able to use a Greataxe for Sneak Attacks?  Would a Rogue find a Whip useful?  How about a Halbred, for a lovely melee ranged Sneak Attack?  Maybe they could use a Maul to really hammer things out.  Maybe they would like to do something at long range?  How about a Heavy Crossbow?  I'm sure that with Sniper, a d10 ranged attack at 400 feet with Sneak Attack would be handy.

So say the stars have all aligned for a Variant Human 4th level Assassin with a 20 dex and the Sniper and Skulker feats.  They have the Initiative, they are undetected, so from 400 feet away, 2d10 +5, 4d6 +5 sneak attack.  That's an average of 41 points of damage.  With a Hand Crossbow, from 120 feet away they get 2d6 +5, 4d6 +5 sneak attack an average of 31points of damage.  It's only 10 more points of damage, but they don't give away their position when firing, so they can keep doing that until someone somehow manages to beat their average of a 19 DC Stealth score from 400 feet away with a perception test and no idea where to look.

 A Rogue cannot use Sneak Attack with a Greataxe, Halberd, or Maul, none of those weapons are ranged, nor do they have finesse. However, gaining proficiency with either a Whip, or Scimitar would be helpful, one for the reach, the other for the possibility to use a scimitar of speed.

As to the heavy crossbow, the d10 damage die is largely irrelevant when the bulk of the damage comes from the multitude of d6s getting added to it. And Sneak Attack specifies that you can’t have disadvantage on the attack, so anything beyond the weapon’s 100-foot normal range could not benefit from sneak attack unless they also took the Sharpshooter feat. But at that point take the Longbow instead. The difference between the d8 and the d10 is negligible and irrelevant, but the additional 50 feet of normal range works even without Sharpshooter, and the added 200 feet with sharpshooter will potentially make a bigger difference.

In fact, IMO the most effective uses for this feat are for Rogues, Monks, and a handful of others to grab proficiency with Whip, Scimitar, and Longbow.

Initially, I thought they maybe had put it in there to leave open the possibility of adding in exotic weapons, but then the feat says simple or martial, so it wouldn't help with that. And so far, we've only gotten revenant blade for "exotic" weapons.

It does make sense though, to allow for a way by RAW to have your wizard use a sword. Even if almost no wizard would really ever take the feat, it does fill a niche that allows for some character customization. I agree with the OP about not understanding why a wizard want to do this, but I don't need to understand why someone makes the choices they do for their character. I mean, if it didn't exist, people would be complaining that you can't make Gandalf without homebrewing his weapon proficiency.

It'd have made a lot more sense if the "must be a simple or martial weapon" clause was left out. Feats are explicitly optional mechanics, so homebrew and/or other optional mechanics could be targeted - firearms, perhaps. My guess it's that it's the result of a slipup in editing during/after development.

Why would you go with a thrown weapon for this? It takes you a whole action to create a new weapon, and if you're willing to just run over and pick it up then why are you not just swinging a sword?

Edit: Are you under the impression that each weapon invocation allows you to have an additional pact weapon? That's not how it works.

In any case, it doesn't matter if you're not proficient with the weapon after it's left your hands because you've already made the attack roll at that point. The only thing you're applying your proficiency bonus to is the attack roll. And that's assuming your DM is pedantic enough to insist that you're not "wielding" a weapon when you throw it.

And on top of all that, the only thrown weapon that a base Warlock isn't already proficient with is the trident. All the other ones are simple and warlocks get simple weapon proficiency. And the trident has the same properties, range, and damage as the spear, so the only time this will ever matter mechanically is if you happen to own a magic trident but not a similarly magic spear.

Back to the actual question, clearly the weapon master feat is aimed by whoever's taken it at their opponent.  Duh.

Lol ok

Given that my own ideas were shot down, and for good reasons, I will offer this.  People with cool charter conceptions.

It's aimed at the people who see characters in fiction and they want be just like them in every detail.  Gandalf uses a longsword, and a quarterstaff in each hand and uses both in the same turn.  That takes a lot of doing.

• People want to play Drizzts du Uden as a Rogue and want to use a pair of Scimitars.
• People want to play Aragorn, but they don't like his fighting style and want a Longsword with Finesse.
• People want to play Tasslehoff Burrfoot, and need stats for a Hoopak.
• People want to play a Space Marine and use firearms.

That's who the Weapon Master feat is probably aimed at.

You see Gandalf "rallying hearts and minds" quite a bit.  Everything from getting Bilbo off on an adventure, to getting Frodo off on his, to getting the Rohimim into the battle in Helms Deep.  He's really good at it.  As a divine agent of some kind, I doubt he needs a ring to do that, I guess it comes in handy though, it's a Focus item, just like his staff.  :-)

And Gandalf may be a ranger (See the thread in the ranger forum)  or a ranger wizard multiclass which would cover the use of the sword ( he is also clearly level 21+ but …)

Quote from Geann >>

Given that my own ideas were shot down, and for good reasons, I will offer this.  People with cool charter conceptions.

It's aimed at the people who see characters in fiction and they want be just like them in every detail.  Gandalf uses a longsword, and a quarterstaff in each hand and uses both in the same turn.  That takes a lot of doing.

• People want to play Drizzts du Uden as a Rogue and want to use a pair of Scimitars.
• People want to play Aragorn, but they don't like his fighting style and want a Longsword with Finesse.
• People want to play Tasslehoff Burrfoot, and need stats for a Hoopak.
• People want to play a Space Marine and use firearms.

That's who the Weapon Master feat is probably aimed at.

 
Firearms are their own special type of weapon and not eligible for use with this feat.

Besides, if you really want to slay alien, mutant, and heretic in the name of the holy god-emperor of mankind: a Battle Master with Sharpshooter, and Crosbow Expert, wearing Adamantine Armor (Plate) and a dread helm, and wielding a Heavy Crossbow with a handful of charges and a burning hands enchantment for a combi-flamer, a repeating hand crossbow as a Bolt Pistol, and a sword of wounding (longsword) as a Chainsword. 

And technically Gandalf doesn’t use a longsword and quarterstaff in each hand, you can hold two weapons in one hand but cannot wield two weapons in each hand. Gandalf used a sword in one hand and a staff in the other. (Sorry, grammar quibble.)

The cleric in the campaign I run took Weapon Master to gain proficiency in martial weapons (flail, greatsword, lance and heavy crossbow) and boost his STR score so he could wear full plate without a speed reduction. 

Quote from Kotath >>

Quote from IamSposta >>

Quote from Geann >>

Given that my own ideas were shot down, and for good reasons, I will offer this.  People with cool charter conceptions.

It's aimed at the people who see characters in fiction and they want be just like them in every detail.  Gandalf uses a longsword, and a quarterstaff in each hand and uses both in the same turn.  That takes a lot of doing.

• People want to play Drizzts du Uden as a Rogue and want to use a pair of Scimitars.
• People want to play Aragorn, but they don't like his fighting style and want a Longsword with Finesse.
• People want to play Tasslehoff Burrfoot, and need stats for a Hoopak.
• People want to play a Space Marine and use firearms.

That's who the Weapon Master feat is probably aimed at.

 
Firearms are their own special type of weapon and not eligible for use with this feat.

 

Citation needed. In the DMG they are a special category of Martial Ranged Weapons. They do need DM permission to exist at all in any given campaign but that is technically true of every weapon. 

Even if Sposta is incorrect, it's irrelevant, because the Gunner feat exists to give proficiency with all firearms.