Both because sometimes I don’t have a phone number. And I don’t want participants to know my phone number.
I don’t get why they have this requirement as it’s not like having a phone number means anything significant. For me, I think privacy includes my ability to not reveal my identity to the network.
They're currently in the testing phase of allowing phone numbers not be known by your conversation partners: https://community.signalusers.org/t/public-username-testing-...