Because you have a Canon camera, a Canon flash, and a Canon off-camera shoe cord, and that's the only flash bracket Canon makes, and only
a Canon flash bracket will do?
Because you love the look of side-cast shadows?
Because you want the flash above the lens in portrait orientation, prefer using the vertical controls on the portrait grip, and happen to be a quadruple-jointed contortionist so that works for you?
See? Three great reasons right there...