Brock Lesnar needs to be the challenge someone needs to overcome. He's the promo guy now, the one who will draw numbers, put butts in seats - he's basically WWE's homing beacon for younger, newer faces, who can come in and take his vaunted spot away from him. We could say he's a new blood magnet that the company thinks will attract enough attention to prevent WWE from becoming a sinking ship.
If he puts the right guy over, Lesnar could fulfill that destiny, that is, if he doesn't get injured before he can give his grand, final performance.