It depends on how much of the work you want Roll20 to do for you and how many options you want. Having the monster tokens pre-filled means that art is in place on them, besides all of their stats being trackable and actions clickable. Having the full roster of ready-to-go monsters is very handy if you like automation or sandboxy games. Having the Players Handbook means that charactermancer (on the official sheet) has full access to the features for character creation, again leading to a fully trackable and clickable sheet. All of the spells, weapons, armor, magic items and more are available with drag and drop simplicity. A lot of this stuff can be done with the free SRD, but owning the books gives you access to the full published options: more spells, more monsters, and especially more character creation options. And of course, the tokens are ready to go. None of the books are necessary , but depending on what you want out of Roll20, they can absolutely make your experience easier .
