I can't tell you why you should.  I can only tell you why I am.

1. I love having access to all my books everywhere I go.  If I am bored and sitting somewhere I can read some of my books on my phone (I have already done this a lot).

2. I drive a lot and can have my phone read my books like audiobooks (I have already listened to most all of Tomb of Annihilation).

3. Everyone in my group can quickly look up rules and spells themselves without stopping the game and flipping through the PHB.

4. I can easily print out handouts and maps.

5. This works on a computer, phone, and tablet.

6. Hyperlinks are incredibly useful.

7. Easy for my players to manage their characters and they can quickly level up right at the table in just a couple minutes right on their phone.

8. Only going to get better and more useful over time.

9. The team a Curse communicates well and listens to all the feedback.

10.  It is instantly updated when new material comes out (if you buy the new material).

11. With a subscription, everyone in the group can share the material they bought with the whole group.

12. The biggest reason is that all of the above work to decrease my prep time as a DM and to speed up gameplay.

You might have other ways of doing similar things and that is fine.  I like DDB and will get my money's worth out of it.   

As mainly a DM, the reason I love DNDBeyond is the ability to search.  I bought the PHB, DMG, MM, SCAG, and PotA adventure (as I was planning on running it).  These are the books I have in physical form at the moment as well.

• Being able to quickly search/browse a spell and bring it up to see it's full text is fantastic.  In Roll20 you can do that with the SRD things, but this, if you buy it, includes everything.
• Being able to search a rule -- I don't do it during a session, like I'm sure most DM's do and many are taught to, I make a ruling at the time.  But then I revisit it later, try to find the RAW, and either make a decision there, or discuss it with my players (was the way i ruled or RAW better for you guys, would you prefer something else, etc.).  Either way, searching the rule via a website is way faster (for me, who isn't super familiar with the books) than paging through the books, or going through the scavenger hunt they call an index (seriously, why have a listing that tells you to go to another listing, the page number takes up less space than the text you're telling me to look at instead).
• Sharing the material with my group, and my family.  My wife recently showed interest but has never gamed at all in anything.  We went through making a character on pen/paper using the books, then we went through again on DnDBeyond, and it was much faster.  It also noted things that we forgot to mark down (i.e. darkvision for her gnome, her AC bonus for being a draconic bloodline sorcerer).  
• A big thing for me is the hope/expectation of a campaign manager -- though that does not exist yet and I personally don't anticipate it until next summer at the earliest.  Note this timeframe is strictly my opinion and has not been mentioned at all -- I'm just basing it on the way things have come out so far, and the timeline/roadmap that they have publicly mentioned to this point.  The campaign manager has been mentioned as something down the road, but is not even included on any roadmaps currently.
• I also like the fact that they aren't killing us with micro-transactions, like they easily could.  You can buy on a granular level, want just a race or class? go for it.  Want a group of spells from one source?  go ahead.  However, buying those things subtracts from the cost of the total of the book, should you decide to purchase it later.  In addition, it also subtracts from the cost of the Legendary Bundle, should you ever decide to go all in and purchase that.  In addition the LB gives you all of the content to that date (so the price increases as books come out, it's more today than it was on day of launch because it includes ToA and XGTE) but a 15% discount on all future purchases through DNDBeyond as well.  For someone who has trouble budgeting, but can spare $2-$4 a week until they pay the $30 for the whole book, and not be penalized for buying it over time is fantastic.  
• The response from the Devs/Product Team have been great.  They have launched parts of the site, through beta and since launch, heard our feedback, and made changes based on that feedback.  They also accept the criticisms from us, and don't just bash us or ignore us.  If you don't like the product, they want to know why and what they can do to make it better for you.
• They give us player-view maps for everything -- that's not really anywhere else.  

As a DM it is well worth it to me. Multiple tabs open to all the monsters, maps, rules, global search, hyperlinks in context, tooltips, etc all add up to a quicker, smoother DMing session for me and I love it. I'm running PotA now and not having to switch physical pages to find a map, an NPC or monster in the back of the book (when combat involves 4 different ones, and some in the MM vs the module) it makes me happy. It also helps me prepare, but there is a lot still to be done on the campaign management front.

I guess it boils down to convenience. I don't see is as buying content again, I am paying for the convenience and related services. e.g. If I buy a paperback i don't get the audio book version for free. A huge factor in justifying the cost was being able to share it all with my players in the campaign. I wish I could restrict the adventures they see, but I understand that is forthcoming. My players help pitch in for the cost of an adventure, so that helps, but honestly I'd probably buy it anyway. There are certainly other hobbies with larger costs, but it's your money and up to you if it's worth it. 

I just need a chunk of change, then I'm buying. It will be awhile but I think the investment is awesome for what I'm getting.

If it were just digital access to all the books through the various search methods and filters, I still would have purchased it.  I used the free compendium throughout the beta, and it saved me tons of time at the table.  The digital character sheet, even in this first iteration, is wonderful on my players' phones.  I haven't run any of the published adventures other than the starter set, but I purchased those in the legendary bundle because of the extra bits (monsters, NPCs, items, etc.) that I can import into my games.  This site has enhanced my gameplay.

Beyond has sped up play significantly for my group. No more passing rule books back and forth between turns to figure out "can I do X" or "how does Y work". It's all right here.

Random encounter time? Just grab the monster(s) I want, jot down a couple of relevant numbers, roll initiative.

I have yet to purchase and use an adventure book here though. I'm waiting the blacklist function for that.

TLDR: Less/faster rules searching = more table and dice time.

i have the the players handbook and volos guide

and will be getting sword coast

 because its useful for making characters for your self and having your players log in and make theres

not sure about the usefulness in the other stuff unless they get some kinda of role 20 alternative 

For me the question is not whether to buy the digital book it whether or not to buy the hard copies?

With digital;

1. All my players can be looking at their own copy at the same time.

2. Errata automatically gets updated into the rules.

3. If I need hard copy I can print it myself. 

4. Search and hyperlink much better than index, and lookup.

Quote from Kryzsko >>

 

I drive a lot and can have my phone read my books like audiobooks (I have already listened to most all of Tomb of Annihilation).

 

I know this message is already a few years old and is out of topic but I was hoping you could assist me on how I could also have my phone read the books on D&D Beyond similar to audiobooks? I definitely want to do this as well.