My favorite part is that it's super customizable, and specifically that it's self-hosted. We ran into issues with Roll20 all the time where it would get super slow or something wasn't working like you'd expect, especially inventory stuff.

I won't say Foundry is perfect, but where the tool itself lacks, the fact that there are thousands of modules that can change functionality or add something cool is just amazing. Modules get made to add blood spatter, deal with terrain, add custom weather effects, add in items from 3rd party books, etc.

And like I said, self-hosting is a big win because we're no longer reliant on someone else. Sure, if the host's internet drops, we can't play, but it's only happened twice in two years of using it.