I have the 3 base books, PHB, MM, and DMG as physical copies. As a DM is there an incentive to buy them again here? The only reason I can find is that I can see monster stat-blocks with MM and magic items with the DMG.

I feel as if though I should get Volo's and XGTE instead unless there's another reason I'm missing?

Getting digital versions of the DMG or MM are basically just to save yourself time with monster stat blocks and magic item descriptions. If you already own them physically I think your best bet is to get XGTE instead. It's got a ton of new content not just for you as the DM, but also for your players that you can share with them online. I'd probably recommend Tasha's over Volo's for the additional options it opens you up to, but that's more if you're intending to share all content with your players.

You definitely want the Player's Handbook or you will be limited to the single option in SRD for character classes.  Being able to click on a Monster and have it roll dice to a VTT has been verrrry useful in our campaigns, but maybe you don't need that.  Of them all the Dungeon Master's Guide is probably the least useful digitally, until you get to magic items where you probably want it. 

That being said, I would have a hard time living without my Legendary Bundle.  It makes everything work great for my groups whether someone is wanting to create an unusual character or searching for items or what have you.  

Thanks so much for the answers! I would looove to get the sourcebook or legendary bundle, but as a student my options are limited. I think I'll get the PHB, XGTE, and Tashas to share with my players. Thanks again! :)

What i would recommend is buy a physical essentials kit (about 20$) and get the free adventures on DnDBeyond and the 50% off Players Handbook (with coupon about $15)   .  Then buy the Monster Manual ($30) and the Magic Items from the Dungeon Masters Guide ($12.99).  Then buy other things as you need it piecemeal  Player wants to play a race in a different book .  Watch for sales and fill in the pieces as you go since the pieces you buy lower the price for the whole product.  You will probably reach a point where its cheaper to use a 20% off coupon to buy the whole book and eventually to get bundles with sales cheaper.  Please note there is almost always a good sale around Black Friday / Cyber Monday.   Check for a coupon they regularly have one on Facebook ads so use that for MM and DMG stuff.

I would think the core 3 with Tasha and XGTE are the 5 best to get first.

For physical books I only have PHB, DMG, and MM

On DNDB I purchased  PHB only.  But one of my players got Eberron for Christmas so I bought that also so she could generate her character based on that book then in order to give them more races and subclasses I ended up getting 4 or 5 more.

I personally wouldn’t buy the MM if you have the book....just the monsters themselves

Besides the PHB, look at the books and consider what parts you really want. You don't have to buy the whole thing, just the sub-classes, or just the magic items, or just the monsters, or whatever.

And the money you spend will be deducted from the book price if you decide later to buy the whole thing.