Sunglasses are great for reducing glare when you are out in the bright sun. But many people still have not discovered the virtues of polarized sunglasses
Polarization is a property given to a sheet of transparent material so that it will only allow through the light waves that are at a certain angle. This is why if you cross two pieces of polarized plastic, the combination will become opaque.
When light bounces off a non-metallic surface at a shallow angle, such as a wet road or your windshield (after reflecting from your dashboard), it becomes partly polarized. Therefore, putting a polarized filter in front of your eyes, rotated to the correct angle, will almost completely eliminate these reflections. This is what polarized sunglasses do.
Sometimes if you're driving with the sun in front of you, the light hits the top of your dashboard directly, then bounces off the windshield and into your eyes. If your dash is lightly colored, this can make it almost impossible to see! Put on the polarized sunglasses, and the reflections almost totally vanish.
I can't even fathom driving in bright sunlight anymore without them.