When you're choosing a username, it's probably a good idea to include something that makes sense in a sentence when people @ you.
Because Mastodon and many other Fedi platforms only show the username by default (and not the server name part), a generic username is difficult to use in discussions.
For example:
It's easy to discuss PixelFed because the account's content is obvious from its username. You can say "Have you checked out @pixelfed? It's a photo sharing platform."
If you use a generic username like "me" or "news" or "updates" or whatever, it tends to cause confusion and doesn't really work when used in a sentence. It sort of forces people to add in brackets what your account actually represents.
Some people with their own servers think their addresses are distinct enough, but because the last part of the address is hidden by default the username needs distinctiveness too.