The addition of a lens hood can help prevent flares--those circles of light that can accompany images where a strong light source is just outside the fame. These types of lens flares occur when indirect light rays pass through the front lens elements but don't make it all the way through to the image sensor. Canon EF 70-200mm f/2.8L USM Lens.
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 dedicated lens hood is designed to match the specific focal length of the lens it was designed for.