Well, it's useful to have local communities, but I personally find it nice to still be able to join, read, and post from another instance without having to make another account. For instance, I'm subscribed to lemmy.world's "local" community, which is where I found out about old.lemmy.world and mlmym.org. Likewise, if I lived in a geographic locality like Seattle, I might want to join a Seattle community on a Seattle instance, but I'd still prefer to be able to do it using an account from another instance rather than being forced to make an account on the Seattle instance.