Do we need roll20 versions of books?

If I or my DM own hard copies (or pdfs) or the rulebooks we intend to use, do we need specific copies of those books for Roll 20 to play them here? Is there anything about the system that requires us to have specific rulebooks owned ON roll 20 to use them in games?

If you want to integrate them for use with your game system, then yes, purchasing the rulebooks will be helpful. If not, you are limited to whatever free resources the publisher provides, assuming they have been added to a Roll20 Compendium. For example, D&D offers the SRD, which has been made available within Roll20. Pathfinder has similar resources. Some systems either do not have a free offering or have not been integrated into Roll20. For example, there is no Compendium for Traveler or Empire of the Petal Throne. If you purchase rulebooks withing Roll20, you have significantly greater resources available, depending on the system. Depending on the game you are playing, there is likely an existing character sheet, which may greatly ease your ability to make characters and play the game. There is nothing preventing you from playing whatever game you wish to play on Roll20. The existence of Roll20 rulebooks makes the experience easier and more automated.

You do not need to own them on Roll20, unless you wish for the automation or quick reference they provide. For instance, purchasing the Monster Manual allows you to easily drag and drop all the monsters, but there is nothing stopping you from building each monster you wish to use by hand. The Player Handbook allows you to fully use the Charactermancer to build and level characters and provides drag and drop for all spells, but you can certainly do all of that by hand as well. Adventure modules are fully built with all monsters and npcs, maps, handouts, etc., but you can build those by hand as well. It basically comes down to how much time and effort you wish to put in. edit: sniped by Keith :)

As a player, I don't think it's necessary. It makes things easier, sure, especially if you use the Charactermancer, but you can still have a fully satisfying gaming experience without a single purchase. The only real benefit you get is that you don't have to fill out your character sheet from hand - it's your call whether that justifies the expense or not. As a DM, there are much more benefits to owning the books on Roll20, though it still definitely isn't necessary. It just saves you a lot of time. I ran games for close to two years using only the hardcovers and the SRD compendium (plus one yawning portal adventure), filling in tokens manually and doing up custom macros for each attack, mainly because I was loathe to buy the books twice. It was certainly more work than having everything I needed in the compendium, but I don't think the games themselves suffered as a result. My laziness has finally overcome my thriftiness, however, and I am now in the process of gradually acquiring the Roll20 versions of the books. But if you don't want to double dip, there's nothing stopping you from running quality games with just Roll20 and your hardcovers.

I ran a 3.5e game here for 4 years, from levels 1 through 18-20, zero Roll20 integration... just a sagging book shelf of source books. Would I have liked a 3.5e compendium... for sure!... did I need  one? Nope. It is purely a matter of convenience for the DM and subsequently their players but certainly not necessary :)

I run games here that do not have any Roll20 books and they are fine. Do I wish that I had books premade and ready to be used as part of the site? Yes I do. Do I need books made into Roll20? No I do not as it is just a convenience and that is all. What you should look at is it more convenient to have the books built into the site to help you run games or does it truly matter if you already have the material on hand and willing to put in the effort to do everything needed manually or close to it.

Thank you all!