Nikon HB-24 Lens Hood
The HB-24 Bayonet Lens Hood from Nikon is specifically designed for the AF-S NIKKOR 80-400mm f/4.5-5.6G 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 get the benefit of extra protection from accidental impact. Dedicated lens hoods are designed to match the specific focal length of the lens it was designed for.