I know people like harking on about phone number or email requirements, but how am I supposed to find my friends? Do I have to send them a message over Signal to tell them the name I picked on another app?
That's the big reason Moxie chose to use phone numbers. Piggyback off the pre existing social network from people's contact lists.
It's great for viral uptake, even if it's not as privacy preserving as a (way way) less usable "random identifier I chose" messenger like, say, Wickr. It's not a surprise that the two big names in all the discussion about WhatApp's Privacy Policy update and mass exodus were Telegram and Signal, with of which use phone numbers as social network kickstarters - instead of "I can choose any username I like but then nobody can find me and anybody could impersonate me" messengers like Wickr.
Not just that though, it makes it much more expensive to say create 1M signal clients to try to spam users, deceiver users, or create a DoS. Phone numbers/sims create a limit on the feasible number of accounts an individual (or even a company) could control.
