So I got a polarizer filter just to keep on the front of the lens and absorb incidental scratches (like a car bumper!), but the COOL thing about a polarizer is that it totally reduces glare. Which means if you are at a museum, you can shoot THROUGH glass cases without getting the reflection of all the other stuff in the room. And if the cases are such that you can walk 360 around them, then you can photograph your friends reacting to art-things like this one:
What a crazy thing! Oh, olden days.