Its easy to generate links to join a server and offers a large degree of control for the administrator. If you want to establish a community forum quickly with the flexibility to change its structure as the community grows its pretty nice.
It has the same draw that the php bbs pages had in the 90's and 00's.
In many Reddit communities you'll see "hey community x, here is my new discord server 'x'"
An extremely niche and obscure (single player) game I follow for example has four discord servers-about one for every three active posters in the community!)
