I think they want discount codes from "sign up to our mailing list!" emails. If it's in the promotions tab, they probably never have to look at it otherwise or get bugged with notifications, but they could still have it there to check when they happen to be making a purchase anyway.

Depending on the email program, you have to open them to unsubscribe, too. But maybe your stats tool would discount opens immediately followed by unsubscribes?