Well, you are not required to use one, but if there are some very good reasons to do so.
What good is the hood? The primary use for a lens hood is to prevent light from hitting the front lens element from the sides – reducing contrast and creating flare. Pictures taken with a lens hood installed can have richer colors and deeper saturation.
A secondary use for a lens hood is to protect the lens. The damage prevented can range from a finger-printed front element (a minor inconvenience) to a broken front element (possibly a major expense – and lost pictures/downtime). Lens hoods are generally strong and stick out from the lens some distance. Accidental touches and scratches to the front element are reduced simply because the glass is more difficult to reach with the hood in place. Some impact protection is gained from having a sacrificial part taking the blow. Of course, ultra wide angle lenses have very short hoods that offer very little protection.
Lens hoods on longer focal length lenses offer more front lens element protection from rain and dust. Of course, if your lens is not weather sealed, you should not let it get wet in the first place. A clean and dry front element will allow the camera to deliver the better image quality you expect.
But lens hoods are expensive – and inconvenient. Yes. But your lens was probably much more expensive – and you are shooting with a D-SLR because you want the best possible image quality (not because it was convenient and inexpensive).
Lens hoods usually attach in reversed position for convenient, but compact, storage. Note that this reversed position is not for shooting and active use of the lens. I am amazed at the number of people I see shooting with their lens hood in this reversed position. Don’t be one of them – the hood is just in the way when reversed – it does no good.
The lens hood can also offer a convenience: I frequently case my camera with lens attached in a toploader-style case or a backpack. To allow a faster shot, I often leave the hood installed (not reversed) and leave the lens cap off – allowing the hood to protect the front lens element.
A small convenience tip: If the lens hood fits too tightly for your liking, add a little body oil to the threads. Your body oil – from your forehead, nose … The hood will be much easier to twist on.
Get in the habit of always using a lens hood!