Lots of amateurs tell me that a polarizer is to be used to darken the sky. They often don’t know that a polarizer’s primary duty is to cut reflections – which is why it darkens the sky.
You can use a polarizer to show the true color of a leaf by cutting through the atmospheric haze that washes out the color in a fall foliage scene. You can reveal what’s hidden behind a window by cutting through its reflection. You can capture items below the surface of a lake, river or ocean.
Polarizers can also do double duty as neutral density filters when you need to reduce the amount of light hitting your sensor.
Just remember that polarizers have limited usefulness when reducing reflections caused by spectral highlights, such as those found on metal objects.