The dedicated Vello ET-86 Lens Hood is specifically designed for use with Canon's EF 70-200mm f/2.8L IS Lens (original vers.).
Lens hoods are primarily designed to prevent unwanted stray light from entering the lens, by extending and shading the end of the lens. A dedicated lens hood is designed to perfectly match the specific focal length of the lens it was designed for. In addition, since the end of the lens is extended, you also get the added benefit of some extra protection from accidental impact.
The "petal" shape of this lens hood maximizes the amount of hood coverage for the minimum amount of weight and hood area.