Joining via server invites that guide you through sign up, no dedicated server to host (I know, major downside for people who don't want all their stuff centralized to Discord's servers), GUI server admin tools, etc.

I think devs tend to vastly overestimate how tech-savvy the average person is. Bring up hosting, DNS, port forwarding, terminal, etc. and they're going to nope out pretty quick. Provide an option that lets you do everything from a single GUI and they'll use it. Enough people use it and eventually the tech-savvy folks have to follow because that's where everyone is.

That's absolutely not to say that it's a good medium for documentation. I will always prefer well-written and organized docs first and searchable forums/issue trackers/SO second. But that second group has a lot of tech elitism and devs who are (perhaps justifiably) short on patience, so Discord seems a lot more accessible to newbies who are asking the most basic questions.