Because it's free, and because it's 2017 :p. But also it's a great way to keep a community engaged. It makes it easier for people to communicate and keep in touch. It's way more convenient than having to use a combination of PM's, exchanging personal skype/IM's, and the site's chatbox. And best of all it has VOIP.
https://support.discordapp.com/hc/en-us/articles/204849977-How-do-I-create-a-server-
https://support.discordapp.com/hc/en-us/articles/204849977-How-do-I-create-a-server-