How e-mail alternates work and why you would set one up:
E-mail alternate addresses are for situations when you expect to receive messages to a specific CU-Boulder mail server, even though you don't have an account there.
The most widely used example is for faculty and staff that used to have mail accounts on spot.colorado.edu. During that time, it's possible that colleagues, associates, and list messages were been addressed to firstname.lastname@example.org. Even though that was a long time ago, address books don't always get updated, so those same colleagues, associates and lists may still be sending messages to email@example.com.
Registering an alternate e-mail address will allow you to receive messages addressed that way. It doesn't mean you actually have an account on that server, it's only a routing rule that ensures your messages will be delivered.