If you can subscribe to communities on other instances than the one you're registered with, why register on one vs another? - Lemmy.world

I am new and trying to understand how Lemmy works. I am posting this from my lemmy.world account, on a lemmy.ml [http://lemmy.ml] community. It seems like you can read, post, subscribe to whatever community outside of the instance you’re registered with. So… Why register on lemmy.world vs lemmy.ml [http://lemmy.ml] or any other instance, if all communities are accessible to everyone?