It's for extra security precautions. This happened awhile ago...Once you do it once, then you don't have to do it again.
http://support.xbox.com/en-US/billing-and-subscriptions/account-security/security-proofs
Proof requirements
Xbox LIVE will require that you have two proofs on your account, and at least one must be a phone number or email address. We recommend one phone and one email proof to provide the best security options for your account.
Why are so many proofs required?
Often people change or stop using a particular phone number or e-mail address. By having two proofs, you are more likely to have access to at least one of these accounts and thereby reduce the chances of being locked out of your account for even a short time period.
starcraft: "I and every PS3 fanboy alive are waiting for Versus more than FFXIII.
Me since the games were revealed, the fanboys since E3." 
Skeeuk: "playstation 3 is the ultimate in gaming acceleration"
