There are many uses for a polarizer filter, but I think the biggest two for me are to reduce glare/reflection in water or glass and eliminate haze.
Reducing glare in the water can make a huge difference to your image. I wish I had a better example for you below. But from the image below you can see why a polarizer can come in handy. The image on the right is without a polarizing filter (*Fun Fact* I actually had the filter on the camera but I didn’t turn it to get rid of the glare)