Lens hoods are an important or integral piece of camera equipment that you must have on all your lenses. The main reason for a lens hood is to prevent lens flare. Lens flares happen when you point your lens toward the sun at just the right angle. It looks like a bunch of translucent rings. Getting lens flares are not super easy to Photoshop out.
What a lens hood does is it keeps the light from entering the lens from the sides and degrading the quality of your image. It also helps improve the saturation, color, contrast, and density of the photograph. If you want to read more on this topic click here for the rest of the story.