A circular polarizer is used with outdoor photography to deepen the intensity of blue skies, reducing glare and cutting reflections. As you can see from the first photo taken from the side view mirror, there is a reflection from what's outside. The second is a photo using the polarizer where it has reduced the reflection. This is also supposed to work with water, too. Here is an example.
The sky is definitely richer with the polarizer. I am looking forward to using it with some sunsets soon.