A polarizing filter screws on to the front of the camera an has two main purposes as far as I have found (for my photography)
- Darken skies to make them look better
- Remove glare from objects or water
#1 can be done in photoshop so it isn't worth the investment but #2 can't. I must say, I don't use this that often but there are times when I want to take a photo of water and it adds to the photo to remove the glare and see what is actually in the water, and a polarizing filter is the only way to do that. I splurged and purchased the Nikon version which was more expensive than other brands because I saw a lot of reviews saying the other ones can fall appart. Also, I bought this with some money I made shooting a wedding in the summer so I was willing to splurge a bit more. Overall I am happy with it so far and hope to find more uses for it this summer when I am up in the mountains taking photos.
To see what this filter does check out the photos I have posted where I used the Nikon 77mm Circular Polarizing II Filter.