Usernames are only for discoverability. You still need a phone number.

You need to install the phone app to be able to activate it. If you've been offline on the desktop app for too long, you need the phone app again to re-active the desktop app. I've also noticed a lot of issues synchronizing messages between computers using the desktop app, without having the app on a phone.

They allow usernames as an alternative to sharing your phone number with other people. You still need a phone number (and the phone app) to create and activate an account.

It's very phone-first.