The Nikon HB-61 Bayonet Lens Hood is for the Nikon AF-S DX Micro-Nikkor f/2.8G 40mm. 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 "petal" shape of this lens hood maximizes the amount of hood coverage for the minimum amount of weight and hood area. This dedicated lens hood is designed to match the specific focal length of the lens it was designed for.