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. The bayonet mount allows you to reverse mount the hood making it easy to store in your camera bag.
Compatible Canon Lenses:
Canon EF 85mm f/1.2L USM Lens
Canon EF 85mm f/1.2L II USM Lens
Canon EF 80-200mm f/2.8L Lens
Canon EF 50mm f/1.0L USM Lens