The Canon ET-67 is a bayonet style accessory lens hood designed specifically for the For Canon EF 100mm f/2.8 Macro USM Lens.
The primary purpose of a lens hood is to deflect unwanted light from entering the lens and causing flare. In addition, a lens hood provides a degree of protection for the lens from accidental impact and from precipitation. A dedicated lens hood is built to fit the focal length of its lens and provides sufficient extension to deflect stray light without intruding into the frame.