This ET-65B Lens Hood from Canon is primarily designed to prevent unwanted stray light from entering the lens by extending and shading the end of the lens. Since the end of the lens is extended, you get the added benefit of some extra protection from accidental impact. This hood is designed specifically for use with the EF 70-300mm f/4.5-5.6 DO IS USM and EF 70-300mm f/4-5.6 IS USM lenses.
EF 70-300mm f/4.5-5.6 DO IS USM
EF 70-300mm f/4-5.6 IS USM