Discord combines a lot of use cases in to one package. You get voice chat, modern chatrooms, video sharing/streaming, direct messages, group messages/calls etc.

But more importantly it operates on a paradigm where a user joining a server means you join all the channels automatically, and access to certain channels can then be revoked or gated instead of granted. This is the exact opposite of what, for example IRC had done (and what Matrix/Element still does to a large extent), and it fosters communities as one group of people can have an n amount text/voice channels dedicated to different conversational topics. This is very useful, even if it's just for a friend group of 5 people. It is no wonder FOSS projects use Discord when it is so useful for it.

Ironically, What Discord does would work incredibly well as a decentralized system. I cannot believe it's taking this long for the FOSS community create an alternative.