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.
The Pentax PH-RBA 52mm Lens Hood is intended for the Pentax smc P-DA 18-55mm f/3.5-5.6 Lens, and is designed to match the specific focal length of the lens. The "petal" shape of this lens hood maximizes the amount of hood coverage for the minimum amount of weight and hood area.