I wear hats and try to cover most of my body when going through thicker brush. I personally don't like when burrs, pricker bushes, or any of the other a-hole plants out there lash out on my head. Hats allow me to charge head first into some of those in order to help shield my face from them as well.
Plus, depending on the climate, hats are good for both keeping me warm and keeping sweat out of my eyes. I also prefer brimmed hats for sun protection since I don't often wear sunglasses while trekking through the woods.
Then, there are a select group of people who always wear hats. As in, they never go anywhere without one on. Whether they just like the feeling or they're trying to cover up some balding, they're a group of people manufacturers aren't likely to forget about.