The "petal" shape of the Dot Line 55mm Snap-On Lens Hood maximizes the amount of hood coverage with the minimum amount of weight and hood area. In addition, the design of this hood allows it to be reversed and stored on the lens, when not in use. It is also designed to accept lens caps.
Lens hoods are primarily designed to prevent unwanted stray light from entering the lens, by extending and shading the end of the lens. In addition, since the end of the lens is extended, you also get the added benefit of some extra protection from accidental impact.