Edit: tldr: I think I probably could've saved myself a lot of time by just saying that discord is like slack but for friends/fun.
I didn't think people use it like lemmy/Reddit. People use it like IRC. That's the analogous tech. IRC is better in almost every way, but not in the most important ways: ease of use, and voice chat.
I know only a handful of people who could set up a server for IRC, but in discord, it's a one-button process. Sure, you can use a public IRC server, but then your channels are harder to organize and you don't have as much moderation control. I dn't think
I would vastly prefer IRC, but even if it was easy to set up, I would still need something for voice chat, and, sure, there are plenty of voice chat tools, but not ones that integrate with text chat so well.
I think a lot of people like the API and the bots built from it, tho personally that's not something I use much.
I'm in probably ~50 servers: groups of friends, video game guilds, tech chat (eg HTMX, Lit, Svelte), random interests (eg mechanical keyboards), and community servers for video games (eg a couple of LFG servers, a couple servers where I can ask questions to tryhards, streamers' communities, etc).
I would vastly prefer to use something FOSS, but there just isn't something that does it so well and so easily -- and even then, I'd probably have to use discord for a bunch of these things.