The Shaper of Worlds has employed the Joker to help him gather parts that are needed to repair himself. Unfortunately for the Joker, one of the labs he breaks into is owned by Bruce Wayne, and employs a scientist by the name of Bruce Banner.
During the robbery, Banner Hulks out, and Batman finds that instead of the Joker, he now must stop the Green Goliath. Wanting to keep an eye on Banner, Wayne keeps him employed, and does whatever he can to keep him calm. Unfortunately the Joker tries once more to rob the place.
This time when Bruce Banner becomes the Hulk, the Joker convinces him, that Batman is his true enemy, and after all, they both have green hair, so they may be related. So while the Hulk attacks Batman, the Joker gets away with what he came for.
So as a way to reward the Joker for his work. The Shaper of Worlds, gives the Joker a small portion of his power to bend reality to his whim.