The main reason projects use it is because a plurality of people already have a discord account, and you don’t need to keep making a new account for every new forum or wiki you want to comment on, read, or post to. I don’t think this is just an issue of “critical mass” ether. Lot of people don’t really want to be handing information to every project they interact with, nor do people want to learn 30 different UIs and quirks.
It’s nature as a chat/call system first has it’s benefits in the form of printed community discussion. People feel like they’re part of a community more easily than on traditional forums and wikis, it’s just more conversational.
It’s far from a good wiki or forum, in fact it is basically non-functional as a wiki, but, as a forum and tech support line, it does work, largely buoyed by the good search function.
It’s open ended enough in it’s functionality, and enough people already know how to use plugins and bots for it, that a lot of it’s short comings can be paved over or overlooked.
It’s bloated and messy and the back end is… yah, and it’s UI and formatting are not well suited for certain tasks. But the average person is far more likely to actually use it. With a single link anyone who already has a discord account can get access to a community, post, comment, and search previous comments and questions. Not to mention that it’s easier to keep track of projects you’re interested in if they’re all centralized on a single platform.