Tilt Shift Lenses?

Need help / education on the benefits of a tilt shift lens. It's one of those things that I just don't know anything about. The lenses offered are quite expensive, and I'm curious as to why they are SO expensive. There must be something there.

What do I / would I get out of having one of these very expensive lenses. I admit, I don't know anything about them. Please educate me on this item.

Cheers