I'm still getting used to roll20.net, both as a player, and a potential DM/GM in the future. The one good thing I can say is that roll20.net is a lot less confusing to me than Fantasy Grounds. Roll20.net is a virtual tabletop that can be used for many, many, many, games, not just Dungeons & Dragons. Additionally you can use hand drawn stick figures, free art, or licensed material that can be purchased.
To me the only reason you would buy anything from roll20.net is if you are a DM/GM and don't have the time/don't feel like, manually setting everything up.
In all honesty the only reason to pay for any of the subscriptions (you can make a free account) is to either A) support the developers of roll20.net and/or B) you want to run games and have better game features for your players, and/or C) you can't stand ads. In my case my reasons were A and C.
I may end up upgrading from the Base to Plus Account if I ever decide to DM/GM a game, just to give the players for features to enjoy, but other than that I just wanted to support the community and also I can't stand ads.
To clarify, by "features" I don't mean content. In addition to the subscription, if you want pre-made/rendered/licensed materials to use then you still need to buy the modules. In addition you can only by modules that provide game content. They don't carry any of the rule books in digital form.