Signals need of your phone number is for two reasons: 
1. Registering an account. This makes it more difficult to create mass amounts of spam/bot accounts.
2. To help people connect. If you already have someone's phone number in your contacts and they also have your number in their contacts, the connection on Signal can be made automatically. However, if you don't want to be found through your phone number, you don't have to be, you can be username only.