What is the acrobatics skill actually used for?

I guess if being prone and having difficult terrain are the only obstacles you ever deal with, then the skill would seem pretty useless. I'll grant you that. But that's not a failing of the rules.

This is a good point, to the OP. FYI 99% of the stuff you will encounter in a real D&D game is not printed in any book. Here's just a list of things off the top of my head of things I have encountered as a player or have run as a DM for my players:

- A glowing magic crystal that works as a steering wheel to control a whole building that can teleport across planes. (required an Arcana check, a Nature check, or a straight Charisma check to use)
- Using those fishing nets on a long pole to try to catch some trouble making teenage pixies (required an Acrobatics check).
- Realizing that the party has been transported forwards in time (required a straight Intelligence check, or a Nature check)
- Jumping from one motorcycle (sorry Devil's Ride) to another in Descent to Avernus to then Shove a demon out of the drivers seat (required an Acrobatics check followed by an Athletics check).
- Making a convincing argument in the Court of Contract Appeals in Hell (this was a skill challenge culminating in an Intelligence (Persuasion) check)
- Quickly placing several items into slots in a puzzle door before a group of living statues could get to them and attack (I asked for an Intelligence (Acrobatics) or Intelligence (Sleight of Hand) check to place all 3 keys in a single Action)
- Teaching a bunch of townsfolk how to build wooden spike barricades before the goblin horde returned (this was Wisdom + Carpentry Tools or Wisdom + Survival against a harder DC)
- Grappling onto then trying to tame a flying reindeer (this was a combo of Athletics checks, Animal Handling checks, and Strength saving throws).
- Convincing a captor that we would never break, and the only way he'd make a profit from us was to send us to the gladiator pits (this was an Intimidation or a Constitution (Persuasion) check).
- Running into a burning house to searching for a way to reach some trapped people without getting too badly burned (this was Acrobatics)
- Trying to quickly launch lifeboats while the ship they were on was in the act of sinking (this was Acrobatics) they had another option of jumping overboard and trying to swim to one of the lifeboats which was Athletics, or of trying to read the waves and dive off and be carried away by the waves using Survival.
- Figuring out how to drive a submarine before we crashed into the ocean floor (this was Investigation or Survival).
- Figuring out an NPCs motivations then appealing to those motivations to convince them to help us (this was Insight then Persuasion with a very low DC)
- Realizing that one enemy was mind-controlling another enemy to direct it what to do and why they were doing that (this was Insight or Arcana)
- Reviving some assassins that had been sedated by a colony of Mychonids after fleeing when the party defeated them (this was Medicine or Nature), then it was Intimidation to get those assassins to cooperate and reveal who had hired them.

Quote from Squirrelloid >>

But in your example... why would it take an acrobatics check to descend the tilted stone?  Either you can move on it or you can't.  If it's difficult terrain, anyone can move on it with double cost, and you yourself says acrobatics doesn't allow you to ignore difficult terrain.  If you would fall prone at the bottom, then you're falling down it?  I don't think you've considered this example very well.  How do the normal movement rules handle it, and why can acrobatics change that?

 Why is this a strict binary? You've never been on a surface where you might or might not fall, and have to fight to keep your balance?

 Either you can move on it or you can't. 

No that is not how the rules in D&D work. Most things you try to do have some chance of success and some chance of failure. It is not the case that either you get hit by a Fireball or you don't, multiple different outcomes can happen if you are in the area of a Fireball, and which one happens is determined by the dice via a saving throw. It's the same if you try to do something. If you try to descend tilted stone safely you might managed it (Successful Acrobatics), or you might not (Failed Acrobatics), but you could also not try to descend safely and end up all scratched up, or you could try brace yourself against the walls and resist sliding down (Successful Athletics) but you might fail and twist your ankle in the process (Failed Athletics). 

Quote from Agilemind >>

I guess if being prone and having difficult terrain are the only obstacles you ever deal with, then the skill would seem pretty useless. I'll grant you that. But that's not a failing of the rules.

This is a good point, to the OP. FYI 99% of the stuff you will encounter in a real D&D game is not printed in any book. Here's just a list of things off the top of my head of things I have encountered as a player or have run as a DM for my players:

- A glowing magic crystal that works as a steering wheel to control a whole building that can teleport across planes. (required an Arcana check, a Nature check, or a straight Charisma check to use)
- Using those fishing nets on a long pole to try to catch some trouble making teenage pixies (required an Acrobatics check).
- Realizing that the party has been transported forwards in time (required a straight Intelligence check, or a Nature check)
- Jumping from one motorcycle (sorry Devil's Ride) to another in Descent to Avernus to then Shove a demon out of the drivers seat (required an Acrobatics check followed by an Athletics check).
- Making a convincing argument in the Court of Contract Appeals in Hell (this was a skill challenge culminating in an Intelligence (Persuasion) check)
- Quickly placing several items into slots in a puzzle door before a group of living statues could get to them and attack (I asked for an Intelligence (Acrobatics) or Intelligence (Sleight of Hand) check to place all 3 keys in a single Action)
- Teaching a bunch of townsfolk how to build wooden spike barricades before the goblin horde returned (this was Wisdom + Carpentry Tools or Wisdom + Survival against a harder DC)
- Grappling onto then trying to tame a flying reindeer (this was a combo of Athletics checks, Animal Handling checks, and Strength saving throws).
- Convincing a captor that we would never break, and the only way he'd make a profit from us was to send us to the gladiator pits (this was an Intimidation or a Constitution (Persuasion) check).
- Running into a burning house to searching for a way to reach some trapped people without getting too badly burned (this was Acrobatics)
- Trying to quickly launch lifeboats while the ship they were on was in the act of sinking (this was Acrobatics) they had another option of jumping overboard and trying to swim to one of the lifeboats which was Athletics, or of trying to read the waves and dive off and be carried away by the waves using Survival.
- Figuring out how to drive a submarine before we crashed into the ocean floor (this was Investigation or Survival).
- Figuring out an NPCs motivations then appealing to those motivations to convince them to help us (this was Insight then Persuasion with a very low DC)
- Realizing that one enemy was mind-controlling another enemy to direct it what to do and why they were doing that (this was Insight or Arcana)
- Reviving some assassins that had been sedated by a colony of Mychonids after fleeing when the party defeated them (this was Medicine or Nature), then it was Intimidation to get those assassins to cooperate and reveal who had hired them.

Fishing for pixies: shouldn't that be a dex save for the pixies, as per the net rules?  (At least in 2024 rules).

Jumping between motorcycles: Considering the rules have no 'inertia' or 'momentum', i'm pretty sure you just dismount one motorcycle and mount the other one (spending all your movement - half to dismount, and half to mount).  If Acrobatics can let you jump from one motorcycle to another without spending movement (or spending less), then it allows you to bypass the mounting and dismounting rules, which is changing the rules.  That's a generalizable mechanical benefit.  If you still had to spend all your movement, then you should never have needed to make the acrobatics check, because you can just do it.

Pushing demon off: At least in 2024 rules, that's a push action, not an athletics check.  (Or you could just hit them with, say, a warhammer when you have mastery in it).

Spiked wood barricades: This was a skill check, really?

Flying Reindeer: pretty obviously played in 2014 rules, as the grapple rules have changed.  Legit for handle animal though, and explicit in the core rules.  This is absolutely what a player would expect to use that skill for.

Burning House: This feels like it should have been a save or take fire damage, based on how other similar effects work.  There's a whole bunch of similar spell effects that you'd get laughed at for suggesting you could make an acrobatics check instead.

Quickly launch lifeboats: I would not have expected this to be an Acrobatics check.  Athletics, if anything.  Of course, I wouldn't expect a medieval ship to have lifeboats, either, but maybe it was a modern setting?

I mean, a lot of the ones I don't mention feel like pretty straight up 'i expect this skill to handle this situation'.  But all the acrobatics ones either feel like you're bypassing actual mechanics in a way that should be generally useable in other situations, or not actually acrobatics at all.

Using acrobatics to cross a space is just like using athletics to climb. There are three common situations:

• You can move without penalty.
• You can move with a penalty.
• You have to make a check, and if successful you can move with a penalty.

Quote from jl8e >>

Quote from Squirrelloid >>

But in your example... why would it take an acrobatics check to descend the tilted stone?  Either you can move on it or you can't.  If it's difficult terrain, anyone can move on it with double cost, and you yourself says acrobatics doesn't allow you to ignore difficult terrain.  If you would fall prone at the bottom, then you're falling down it?  I don't think you've considered this example very well.  How do the normal movement rules handle it, and why can acrobatics change that?

 Why is this a strict binary? You've never been on a surface where you might or might not fall, and have to fight to keep your balance?

Quote from Agilemind >>

 Either you can move on it or you can't. 

No that is not how the rules in D&D work. Most things you try to do have some chance of success and some chance of failure. It is not the case that either you get hit by a Fireball or you don't, multiple different outcomes can happen if you are in the area of a Fireball, and which one happens is determined by the dice via a saving throw. It's the same if you try to do something. If you try to descend tilted stone safely you might managed it (Successful Acrobatics), or you might not (Failed Acrobatics), but you could also not try to descend safely and end up all scratched up, or you could try brace yourself against the walls and resist sliding down (Successful Athletics) but you might fail and twist your ankle in the process (Failed Athletics). 

All outcomes are binary - either success or fail.

The movement rules define moving through space.  So either you are moving down this sloped stone in some defined way, or you're falling.  In either way, the rules dictate the outcome.

If you're saying Acrobatics allows a different outcome not covered by the normal rules, then Acrobatics is allowing you to break the rules on a skill check.  And the way in which it does that should be generally applicable.  But first you need to properly define the problem.

• If the slope is difficult terrain, then acrobatics is letting you bypass difficult terrain.
• If the slope is too steep to move across normally, then you're falling, and acrobatics is allowing you to ignore damage from a fall, and should generally allow you to ignore (some) damage from a fall and avoid being prone.

And you figure out which by figuring out how a person not using any skill navigates the terrain.  If they just tumble down the slope, it's falling, and they should take fall damage.

Now we've quantified the mechanical benefit acrobatics is giving you.  But... the rules nowhere provide for that benefit.  This is allowing acrobatics to break the rules.  And any such use you come up with for acrobatics is going to be allowing you to bypass some rule that governs movement in some way, if for no other reason than acrobatics allowing you to traverse space in a way you normally can't do is, by definition, making an exception to the general movement rules.  This is tautological.

(I certainly agree it should allow you to break the rules like that.  But it shouldn't be unique to that slope - acrobatics should always let you do that).