Consider the following situation: Alice is a hand-held device, such as a PDA. Bob is a device providing a service, such as an ATM, an automatic door, or an anti-aircraft gun pointing at the gyro-copter in which Alice is travelling.
Bob and Alice have never directly met before, but share a key as a result of secure hand-offs. Alice has used this key to request a service from Bob (dispense cash, open door, don’t shoot). Before complying, Bob needs to be sure that it really is Alice that he can see in front of him, and not Mort.