To Reduce Glare and Reflections
These photographs were made with a polarizing filter attached to the lens. For the photo on the left, the filter was set for minimum effect, and for the one on the right it was rotated 90 degrees for maximum effect. As you can see, the polarizer eliminated almost all the glare from the water. Not only did the polarizer remove the glare, but it also removed the reflections. In the photo on the left you can see the reflections of the trees and sky. The highlights in the water are not actually direct sunlight, but are reflections of the overcast sky. In the photo on the right, the water is clear and you can see the bottom. Dialing in the polarizer also eliminated the shiny highlights from the leaves and from the scales of our little reptilian friend.
A polarizing filter will reduce the light reaching your sensor by a stop or two. In that regard, it can perform as a neutral density filter, should you so desire. Some folks are so enthralled by the wonderful things their polarizing filter can do that they keep it mounted all the time, but in most cases I think one should remove the filter from the lens if it isn't needed. In this example, dialing in the polarizer reduced the light by another full stop. Both photos were made at ISO 400 and f8, using aperture priority with +1/3 stop of exposure compensation. The exposure time for the photo on the left was 4/10 of a second and 8/10 for the one on the right. It was an overcast day, deep in a cypress swamp – not enough light for hand-holding, especially so with the filter attached. (Even though the camera was tripod mounted, I'm ashamed to admit that I did not use proper slow-shutter-speed technique, e.g. mirror lock-up and cable release, for these photos and so their sharpness is wanting)
The handsome creature on the log is a cottonmouth water moccasin, a venomous snake. In my experience, the reputation of the cottonmouth for being aggressive is greatly exaggerated.