As someone who hit the PC gaming world in around 2008 online, we started with TeamSpeak and Ventrilo which were hosted platforms that allowed you to voice chat with your friends. They had cost money and weren't that great on audio quality (from that time).

Then we went to separate game specific voice chat platforms which were inconsistent, but meant you struggled to find a way to chat with mates outside of something like Skype at the time.

Discord was one of the first platforms that was firstly free and allowed you one place to chat with friends and communities about games AND had very good quality audio.

People use it for all sorts of things, however I use it fairly minimally and only to be able to voice chat with friends either when gaming together or playing separate games. I don't get understand some peoples problems with it, it works better than anything else out there for what I use it for.