Much like why some people contribute to open source projects, sometimes people find a thing useful but think it could be better, and spend some of their time to

Much like why some people contribute to open source projects, sometimes people find a thing useful but think it could be better, and spend some of their time to make it so, either by helping to organize things so the subreddit isn't frequently the same 6 posts, or organizing community recurring events, or a million other things to foster a community. There's also plenty of people who like the authority, or a thousand other reasons, but that's one example.