The dedicated Lens Hood is specifically designed for use with Nikon's 18-55mm f/3.5-5.6G VR lens. It is primarily used to prevent unwanted stray light from entering the lens by extending and shading the end of it. A dedicated hood is crafted to perfectly suit the specific focal length of the lens it is designed for. Additionally, this hood features a non-glare matte finish on the interior surface to further prevent or limit lens flare. Also, since the end of the lens is extended, you get the added benefit of some extra protection from accidental impact. The "flower or petal" shape of this lens hood maximizes the amount of hood coverage for the minimum amount of weight and hood area. The bayonet mount is designed to also work in the reversed position, allowing the hood to be placed backwards over the lens for a more compact storage solution.
This is Nikon's official lens hood for the Nikon 18-55 F/3.5-5.6G ED II AF-S DX lens, which is the kit lens that many Nikon cameras such as the D5100 come bundled with. This hood allows the lens cap to be inserted onto the front of the hood, while the hood itself attaches to either the lens, or more likely the UV filter screw threads. Since this hood screws onto the UV filter rather than attaching to the lens bayonet style, this means that the lens is not designed for quick removal. It is also not designed to be quickly removed and then reversed onto the lens for storage in a camera case, such as my Pelican bag. This is mostly OK, as this is only a 52mm hood and thus is relatively unobtrusive; just be aware that this lens hood is not really designed to be frequently removed and thus it does add about an inch to the length of the lens.