So you can use more than one iPhone as your primary phone. For example, if you have the new iPhone Duo but want to use your other 'standard' iPhone for the telephoto zoom camera capability, you could transfer your phone number from the Duo to that phone for the day if you're going to be out taking a lot of pictures. When you are done, transfer it back to the Duo, and the Duo will become your primary phone again.

Travel is one. Have a daily driver phone that you use at home, and another one with much less personal data that you take across borders.