[vehementi]

The historical context of discord, is that every gaming group would have some sort of chat app (mainly IRC, more recently hipchat, yammer, slack) as well as a push-to-talk voice app (ventrilo, teamspeak, mumble, ...). Discord is merely combining the best of both (slack, and all its api support, with built in voice channels).

Nobody has to use voice. In many communities, most don't. But as the above said, it's a super frictionless way to talk if needed and is a lot less formal than "starting a call/meeting". You just publicly hop into one of a dozen visible channels and anyone else can hop in with you to discuss an issue or just hang out mostly-not-talking.