A good tight end is big and he can both block and catch. He's an eligible receiver, able to catch passes in heavy traffic because of his size. He's bigger than a wide receiver, but faster than a lineman.
On any given play, the tight end could block or could go out for a pass. Throw three tight ends in there on the same side of the line and you have a nightmare for the defensive coordinator. It could be the next big step in the evolution of offense. Also: It's a 3-4 defense killer.