When light bounces off a flat or mirrored type of object (think glass, smooth water, car hood, etc) , the waves tend to become oriented the same way. Polarized glasses block out this type of light.
Even with dark sunglasses, bright glare from a windshield could be too bright to look at. Polarized glasses will help with this. They also tend to let you see into bodies of water, rather than surface reflections.
Careful though, sometimes polarized glasses can cause problems with certain LCD displays (for example there's always a chance you won't be able to see your car radio/clock/whatever display with them, depending on the display).
Overall polarization is a great advantage I think, but it can cost a lot.
BTW in response to the original question, Oakleys or Ray Bans