Signal is adding usernames to protect visibility of phone numbers. Usernames are effectively a privacy work around, not a social feature:

Signal usernames are not logins or handles that you’ll be known by on the app – they’re simply a quick way to connect without sharing a phone number.

They also go on to describe that they’re easily changeable/disposable to share with different groups over time.

The Signal Wiki documents the generation of nicknames and UUID-based links (links don’t publicly display your username)