signal requires a phone number to sign up. a phone number could be used to trace your signal account back to you. so why do people, especially privacy enthusiasts and experts (like edward snowden), still use it and endorse it when it lacks anonymity in that sense? i get that people could use a voip number or something to sign up, but still.
Also while it seems ideal to have both (don't know who lives there or what they're up to), it's much harder to implement in a way that there aren't ethical/legal headaches.
It can also be easier from the usability side, like if you want to get in touch with your friend and you know where they are living today.
I'd like to have the option to not use a number, and it would help those that live under authoritarian regimes, but I don't think I'll go through the trouble of removing mine once they implement it
As far as messangers go, there are plenty which don’t require a phone numer, like Threema.
So for this instance, it is pretty easy to have both