One of the most important accessories for each lens you own. A lens hood provides multiple functions: it shades the lens from stray light, improving your contrast and image quality; in inclement weather, it can assist in keeping moisture or wind-blown debris off the lens; and it protects the front barrel from the inevitable impacts against walls, door frames, and other real-life obstacles. Almost all EF and TS-E lenses have a Canon lens hood engineered specifically for it.
The addition of a lens hood can help prevent flares--those circles of light that can accompany images where a strong light source is just outside the fame. These type of lens flares occur when indirect light rays pass through the front lens elements but don't make it all the way through to the image sensor. Canon's EW-73B Lens Hood is designed for the Canon EF-S 17-85 f/4-5.6 IS USM SLR Lens.