The dedicated Nikon HK-33 Lens Hood is specifically designed for use with the 400mm f/2.8G ED VR lens.
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. This dedicated lens hood is designed to match the specific focal length of the lens it was designed for.