I really dislike that about the app too. In fairness, I think the goal was to replace Apple Message and SMS which require a phone number but it is also self defeating in a sense because it breaks the privacy and plausible deniability Signal is supposed to offer by tying an app instance to a particular piece of hardware.

Almost every communication app requires a phone number now. The claim is that this limits spam which is probably true but I suspect there is tremendous value in having real phone numbers and the ability to do pervasive tracking. We know mobile phone providers sell data on phones. The best that can be said about the Signal org is they are non profit but this is not very reassuring especially given the threat model of people who might be using Signal.