I agree with you: I would rather not give my phone number to companies either. But once you start to see spam or get spam messages from other users (imagine a

I agree with you: I would rather not give my phone number to companies either. But once you start to see spam or get spam messages from other users (imagine a dating site) you would like this spam to stop. Having a (unique) phone number a mandatory requirement during registration can make spam fighting much easier.