You should own and use a lens hood on your lens for two big reasons:
Flare: This occurs when light strikes the front element of a lens and is then scattered through internal reflections and within the elements themselves. Flare makes its presence know in visible artifacts or as an overall haze across the image making it look “washed out.” There was a (misguided?) time in my photographic past when I sometimes used flare as a compositional element in an image. (I guess it was the latent J.J. Abrams in me, although at the time I didn’t know who he was.) These days I don’t feel- the same way.
Protection: Unlike some of my photographic compadres I’m pretty careful with my gear but accidents happen and banging a lens against an immovable object can by mitigated by a lens hood. Hey, it’s happened to me one; never again. A hood can be cheaper than a lens repair or can it?
Back in the good ole days of film photography every lens came with a lens hood. And not just any hood but a real jumping-up-and-down screw-on metal lens hood. Nowadays most camera companies make lens hoods optional and expensive. To be fair, many lens companies, such as Tamron, include a lens hood at no extra cost. (I just tested Tamron’s 85mm f/1.8 Di VC USD lens, look for a review on Shutterbug’s website real soon now.)
And if your main objection to using lens hoods is that they cost too much I can only add “I feel your pain” but I also have a solution—third party lens hoods. Take one of my favorite mirrorless lenses, the Olympus Zuiko Digital 45mm f/1.8. It’s hard to beat for portraiture and I love everything about this lens except the cost of a lens hood. The official Olympus LH-40B hood costs $34.15, while the Vello LH-40B version is only $19.95 and they look a lot alike. But wait there’s more. You can do what I did and get a JJC LH40B hood for ten bucks.