This may be totally off base, but I believe the cards, when used properly, give us insight into problems, and gets us to look at things in a different way. It can even reveal hidden things (or, rather, things we don't want to face), and brings the subconscious out into the open.
People use it for all sorts of reasons, mainly for advice, or to help them solve a problem from a different angle that they may not have thought of. Or, even to reveal the root of a problem that keeps propping up. Sometimes people use it to predict the future (like "will I ever get married" etc), but I don't believe that spreads like that are completely effective.
Actually, I think that's the difference between prophecy and divination - prophecy is about predicting the future, while divination is about insight into the past and present, and only helps us prepare for the future.