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.

Large Square Lens Hood for 21/4, 25/4, 28/3.5 Leica screw mount lenses, 
or 21/4 or 25/4 M mount lenses, NOT the 28/1.9 screw mount lens,  
will also fit the 35/2.5 C screw mount lens, but not the 35/2.5 Pancake screw mount lens
nor the 35/2.5 P M mount lens