If I don't use Signal, then I'm not going to keep a list of my friends' Signal usernames on my phone. If I subsequently sign-up for Signal, then I have no way

If I don't use Signal, then I'm not going to keep a list of my friends' Signal usernames on my phone. If I subsequently sign-up for Signal, then I have no way to discover which of them use Signal - short of contacting them via some other method and asking for their Signal username, if any. By making the Signal username the same as the user's phone number, I actually DO have a list of Signal 'usernames' on my phone already. As soon as I sign-up, I can send my list of friends' phone numbers to Signal and they can tell me which of those people have Signal accounts.