To validate that a user is a person. The idea is to trust the phone companies that a person who happens to possess a phone number is actually a person.
I never said it was a good solution. There is no way to trust any validation that a user on the Internet is a person. But this way is cheap easy and most people aren't gonna go through the effort of masking their identities.
Also one discrepancy in an audit of a phone number trusted user base sticks out enough for cops to make some progress.
People are putting too much thought into this. It's discovery. Signal is a WhatsApp alternative. You switch from WhatsApp and want to know which of your contacts you can still talk to? No action necessary, you can do it right away.
I guess that's true, but I'd prefer the phone number part being optional. If you don't give it, you don't get access to the easy migration or discovery features, but you get to hide your phone number.
You need some sort of verification that the person is a person. Phone number puts a layer between you and the service you are trying to use - the provider of the number. The provider holds your identity but only passes on a phone number.
Thing is it is very easy to get a signal account with a fake number, I have 3 different ones. My spare phone don't have a sim or number, but do have signal. On my main phone I have one for each profile.
Privacy and anonimity are different things. As long as nobody besides you and the indented destination(s) has access to the content of your communication, that communication maintains privacy, even if everyone sees that it's you talking.
Also, and this is something I mention all the time, the only information this gives is that you use signal. Besides that, as soon as anybody else registered your phone in their contact list, your phone number is already known and associated with you considering that many apps (like all the meta ones) gain access to the contact list and the chance that anybody who has your phone number uses one of those is almost 100%.
App-accessible contact lists is the original sin of smartphones. As a result, a few powerful corporations know the social graph of entire countries. The handful of people who make efforts to stay anonymous be damned - they're in the database too thanks to their friends. This one infuriating feature makes decent privacy all but impossible.
They do their best to use the number in ways no one but your contacts who use Signal can actually see what that number is, to be fair. And you're still private either way. What a phone number breaks is anonymity, which is something they don't explicitely claim to give you. (I think)