Lens hoods are designed to prevent any stray light from entering the lens. They do this by extending and shading the end of the lens. When using a lens hood, the end of the lens is extended, so you also get the added benefit of some extra protection from any accidental impact.
Dedicated lens hoods are designed only to match the specific focal length of the lens that the hood was designed for. This hood is for EF 75-300mm f/4.0-5.6 USM, II, II USM, III & III USM Lenses.