Why does Google ask for your cell phone number when registering a gmail account?

It was odd and a new tactic by a company...why such detail required?
Why does Google ask for your cell phone number when registering a gmail account?
It allows them to restrict it to one account per person and a way to verify that the phone number is real and yours. I believe you have to respond with a code that appears on the phone.
Reply:They want to know whether, on your gmail logon screen, they should set you up to offer "gmail mobile"
Reply:To prove you are a real person, not a computer randomly signing up for fake accounts to spam with.