The dedicated Vello EW-78E Lens Hood is specifically designed for use with Canon's EF-S 15-85mm f/3.5-5.6 IS USM lens. It is primarily used to prevent unwanted stray light from entering the lens by extending and shading the end of it. A dedicated hood is crafted to perfectly suit the specific focal length of the lens it is designed for. Additionally, this hood features a non-glare matte finish on the interior surface to further prevent or limit lens flare. Also, since the end of the lens is extended, you 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. The bayonet mount is designed to also work in the reversed position, allowing the hood to be placed backwards over the lens for a more compact storage solution.