Why should I have a phone number associated with my Twitter account?
Allowing your friends to find you (if they have your phone number saved as a contact).
Or perhaps someone ELSE who ‘wants to find you’...
You can use an email instead of a phone #. For setting up many accounts, an email is sent with a link to click or a text to your phone with a ‘code’ you have to enter to finalize setting up your account. If your account is hacked, there is a verified way to be in touch with you/account owner for recovery purposes. That’s my best guess.