Comment by marssaxman

What they need it for is simply that it's the way the system has always worked, because Signal started life as an encrypted replacement for SMS. The point was that you could switch from the standard SMS app you were already using over to Signal (which was called "TextSecure" at the time) without having to change your habits, because sending messages to people's phone numbers was simply what people did then. There's nothing nefarious about it.