When is a Swivel not a Swivel?

The swivel is normally the tie point between line and leader. It is supposed to act, as its name implies, as a swivel point. Essentially, the leader and everything below the swivel can spin and twist with out twisting your line. Instead, the bottom half of the swivel is supposed to turn. We all use them at some point in our fishing life. Swivels are an important part of the terminal tackle in use by most saltwater anglers. Yes, we all use them - but do we all use them for the same reasons?