It isn’t at all a new concept and I’m not sure why people coming from reddit continue to get stuck on it.
Because having communities with an identical name on different instances will fracture the community. Given the hallmarks of the fediverse this is practically intended, to my understanding, but it is bad for initial growth and coherence of posts. This happened on Reddit as well, of course it did, but the way instances are completely separate and communities can have the exact same name compounds the issue.