Lets say that at some point you decided to adhere to security best practices and set a password on your iPhone backups so that they are encrypted. A year or two later you have upgraded your iPhone to a new version and you want to transfer all of your data across to the new phone. You attempt to restore from your backup and, doh, you need to remember the password you set. You try every password you could have set but none of them work.
SMS 2-factor authentication has been implemented by a number of security conscious organisations, including banks, to secure online transactions. SMS 2-factor authentication has had a major impact in reducing online fraud. This is because an attacker most not only capture the victim's username and password to login to their bank account, but they must now also have the victim's phone to receive the SMS 2-factor authentication token. This restricts the number of possible attackers dramatically.