As a few other people mentioned, this is only meant for the unorganized “Welcome to Discourse” pseudo-category. I imagine that when our own Eternal September comes, this is where people will say “Hi” and ask for homework assignments to be pasted.
More boring context
Each Discourse instance has this sort of catch-all, called Uncategorized by default. I renamed ours, somewhat against recommendations of the Discourse devs, because I don’t want us to invite uncategorized posts. I recently learned this pseudo-category can be disabled entirely. So I will do that soon and move the “Welcome to Discourse” posts to an actual category of the same name, then it will be obvious if somebody posts there (since there will be a category badge).