Recommended content for 5E

Looking to start a campaign soon. Going to do Waterdeep Dragon Heist.  Also love the idea of using Beyond and its extensions. Plan on buying the Campaign for Heist and likely the monster manual for all the tokens and such, but what am I gaining over SRD and Basic Rules by getting the Players Handbook? Just trying to reduce having to buy a third copy of something I already bought twice. Once physical and once on Beyond. 

There are many spells that are unique to the non-core casters (bards, druids, paladins, warlocks) that do not appear in the SRD. You also don't get more than 1 subclass for each class or feats in the SRD. You don't need to have any of those in Roll20 to be able to use them, you would just need to do a lot of manual entering, but if you have D&D Beyond you could probably just copy/paste the stuff you need.