The used HB-23 Bayonet Lens Hood from Nikon is a petal-type bayonet mount lens hood for the following Nikon wide-angle zoom lenses:

• Nikon 17-35mm f/2.8D ED AF-S IF
• Nikon 10-24mm f/3.5-4.5G AF-S DX
• Nikon 12-24mm f/4G AF-S DX
• Nikon 16-35mm f/4G ED AF-S VR

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.