This Leica Rectangular Metal Lens Hood with Cap is specifically designed for the 75mm and 90mm f/2.5 M lenses. It comes supplied with a custom cap that is especially designed for this hood.
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.