On its 10th anniversary, Signal’s president wants to remind you that the world’s most secure communications platform is a nonprofit. It’s free. It doesn’t track you or serve you ads. It pays its engineers very well. And it’s a go-to app for hundreds of millions of people.
I think another reason they use a phone number is that it can mitigate issues with people or bots creating hundred of accounts maybe
But there are plenty of other services that don’t require a phone number that also seem to mitigate that issue, so while it may be a convenient option, it’s hardly the only option.