Why is that? Not disagreeing but genuinely curious. One of the issues I face sometimes is wanting to stay in touch with someone I met. I'd like to do that over Signal (I don't use any of the popular social media platforms) but I don't feel comfortable sharing my phone number with them. It would be nice if I could use a unique user handle instead.
It allows Signal to work similarly to the messaging services it replaces without having to keep serverside contact lists. Those contact lists, which practically every other "secure messenger" keeps, are the most valuable metadata the service keeps, in many cases more than the content itself: they're a record of who talks to who. Signal's phone number system means they can keep those contact lists clientside by piggybacking on the device contact list, which is keyed by phone number.
