There are a few purposes that I love Discord for, but they basically all summarize into "chatting with my friends about shit." On top of ALL of the reasons above, which all resonate with me, there's some additional stuff as a creator and a critic. Seeing everything get walled off there has been brutal.
When I was in media, being asked to join a discord to get access to a game for coverage was always a net negative. Multiple times it led to weird shit from other members of the discord in question, either fans who had an axe to grind with me or peers in the industry who saw this as an opportunity to be weird about shit. More importantly: It really changes the power dynamic between publisher and critic in a way that's hard to quantify but the sense of attempted critic capture is very real. When you go and check out a game at an event, then leave it and share your thoughts, things are temporally discreet. In a discord, you're just like... there. And you know, the second a PR person shoots you a friend request, it's like, welp...
As a creator, it might be even worse. Discords can create a really incredible feeling when you release a project, similar to how a live chat reacts to a great moment during a stream. But the trade off is closing that moment off behind closed doors. Even just in the case of FatT, for instance, there was a time when a new episode drop led to a flurry of posts on Twitter and Tumblr. When we launched our Discord (which we've all now left at this point), and especially as that Discord grew, that activity took a nose dive. It became the place for the most active members of the community to talk--and as such, a lot of our word of mouth growth really dropped off. (This is part of why things like Secret Samol have been so incredible for us!)
Obviously, there's a lot of other stuff here (which I also have big biases about) around how conversations Work in the closed, fast-moving sphere of a Discord versus the more draft-and-send open air of something like G+ (RIP) or Tumblr, but that is more than I have time to unpack during the work day.