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.
The B+W #960 metal telephoto lens hood is made of black-anodized aluminum with light-absorbing flat black ribs on the inside, and due to it's cylindrical shape this hood is especially suited for telephoto lenses.