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.

This Canon EW-78BII Lens Hood in particular is deigned for use with the EF 28-135mm f/3.5-5.6 IS Lens.

The "petal" shape of this lens hood maximises the amount of hood coverage for the minimum amount of weight and hood area. This dedicated lens hood is designed to match the specific focal length of the lens it was designed for.