TL;DR: to allow users to make contacts without revealing their phone numbers, but avoiding creating new surfaces for harm (spam, abusive usernames) in the process (which also eliminates all the issues that username moderation would bring).

Why Signal ‘turned our architecture inside out’ for its latest privacy feature | TechCrunch

“The user name is not a handle. It’s not shown inside the app; it’s not something we have a directory for. But it replaces the phone number when you go to initiate contact.”