Nothing, really. Each instance is unique. Technically speaking, users will the same name on different instances are unique. You can always spin up your own instance and get whatever name you want. So it's natural to want to register a username in multiple instances, to avoid confusion. Eventually there will have to be some way to tie identities across instances. Like a GPG key that can be validated on all accounts belonging to the same person.

I think this is ultimately where Meta is going with their Fediverse stuff. They will operate as a low-key ID verification service. Instances are saying they won't federate with Threads right now, but as shitty as Meta and Instagram are they can guarantee that a Threads user is the same entity as an Intstagram user, which is currently more ID verification than exists right now in the Fediverse. Some instances may open themselves up to Threads in order to gain access to their userbase.