It's not email addresses they want most dearly though (although it is used and correlated of course) it is that nice unique identifier that is your (mobile) phone number. For smartphone apps it is the default (and often only) identifier (and conveniently, it is also the one identifier nearly always present in the contact lists they guzzle in).

A lot of services on the web want your phone number too; as a contact on the shipping label (often a required field), as part of the authentication scheme (out-of-band text messages with OTP codes to confirm certain actions), or as an account recovery option. A person's phone number is a unique numerical identifier that doesn't change often, if at all (in the EU you tend keep your mobile phone number when switching telco's).