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 #920 wide-angle rubber lens hood is made of a high-quality matte black rubber with a black anodized aluminum filter ring, and folds down and protects against contrast-reducing, uncontrolled, stray light. It is designed for wide-angle (zoom) lenses with angles of view up to 70° wide (approx. 28mm in 35mm format).