The dedicated Nikon HB-45 Lens Hood is specifically designed for use with the AF-S DX NIKKOR 18-55mm f/3.5-5.6G ED II or 18-55mm f/3.5-5.6G VR lens. It is not compatible with the 18-55mm f/3.5-5.6G VR II, which uses the HB-69 hood.
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.