Why does facebook ask for your mobile phone number?

facebook keeps asking me for my mobile phone number, (but what if someone didnt have a phone number?????) it wouldnt let me log in unless i gave it to them so i did, and gave it the code it texted back then it sayed verifyed. and every time i log in it says the words your account is already verifyed! nothing else , i cant check my updates or mail. any one else having the problem ? if so pleases tell me if u have a solution, thank uuu