Offering usernames will allow people to stop giving out their phone number —a sensitive piece of information— in order to connect with others on the messaging app.
Yeah that's Android only. If you drop your iPhone in a toilet, or if you need to factory reset it for any reason, everything in signal is gone. Even if you backup your device with the system backup feature, Signal sets a flag on all the files it writs to disk so they are excluded from all backups.
It's been the number one feature request on their iPhone community support channel for six years, and the official response is "We will probably never add that feature. We understand that's frustrating. Stop wasting our time by asking for it". Meanwhile every day someone, somewhere, loses all of their message history.