Why does Google require account holders to register a phone number?

[oxmounter]

One seems to need to register a cell phone # in order to set up a Google account.  What about folks who don't HAVE one?

[rcrawford34]

you don't have to register a mobile phone #,  it's optional

from google

Mobile phone. Having a mobile phone number on your account is one of the easiest and most reliable ways to help keep your account safe. Since you have physical possession of your phone, we can provide easier ways for you to get back into your account if you ever forget your password. And we can use it to challenge unusual accesses to your account. This field is optional, but strongly recommended if you have a mobile phone.