It certainly scales like shit, but Discord has a very smooth text chat/video sharing features that work extremely well for smaller numbers of people. Like for me and a dozen friends it is the perfect social space, but anything bigger than that and I bounce off hard.

It's incredibly simple. No one has to host a server, and it works well enough for everything you might try to use it for. Try to get someone to use something different, like teamspeak, mumble, ventrilo, or the copycats akin to matrix, and you will get endless bitching about some little thing that doesn't get done (usually screen sharing).

Iirc it evolved from Teamspeak to facilitate/coordinate game play

Ease of use and screen sharing. It's incredibly easy to share your screen with others in the call/room which I haven't seen replicated to the same quality elsewhere.

I have a lot of other issues with discord though, I strongly dislike that people have started to use it as a replacement for traditional forums