I'm using Lemmy.world, but I still need to get away from the traditional mental model of usernames. It bothers me a bit knowing someone could use my username on another server and interact with the servers I do... But then again, nothing prevents someone from creating my Gmail on Yahoo. So it's just a matter of getting used to things.
You could always create an account for your username on multiple popular servers if you wanted to. Can't stop it on small servers but having multiple accounts in the Fediverse is not a crime.
Just if you wanted more than one. Just like u/kadu said about email. It's like if you had your email on Google, Yahoo, and Hotmail. I know a lot of people have an alt on platforms like Discord too.