Right. The key thing is that the “Public Domain”, and dedication thereto, doesn't exist in all countries, or at least, it doesn't always function the same way

Right. The key thing is that the “Public Domain”, and dedication thereto, doesn't exist in all countries, or at least, it doesn't always function the same way as it does in the U.S. So CC0 tries to get as close as possible in such countries, by waiving all applicable rights and so on, or if that's not possible, giving a very liberal license grant.