IMO its a good competitor to slack. They probably make money from businesses too. They have lots of options for permissions/roles and all kinds of API access to write bots for.
0: https://support.discord.com/hc/en-us/articles/360028038352-S...
Think: every content-creator or streamer.
But also: regular corporations that provide platform services that people build their own stuff on top of, such that people want to talk to each-other about the service rather than just talking to the corporation about the service. (The sort of thing you used to stand up a hosted forum for.)
Unsure what their billing looks like, but it works pretty well for them apparently.