Typically most discussions surrounding the use of polarizing filters center on how they are used to eliminate reflections.  Another less often mentioned benefit (actually just an implication of removing the reflections) is increased color saturation.

While setting up for a waterfall shot, I noticed people walking underneath and tried to grab a quick shot to show scale. I hadn’t yet finished composing or setting the polarizer. The left-hand shot shows how the scene looked w/o a polarizer (it was on the lens, but not rotated to the correct angle to get any benefit). Reflections are evident on the leaves and on the rocks at the top and bottom of the falls.

The right-hand image from less than a minute later is the same scene (zoomed in a little) and same exposure with the polarizer adjusted for maximum effect. The most obvious difference is on the foliage, where the removal of the reflected sky really brings out the color, and thus increases the color saturation in the image. Less obvious, but still helpful, is the removal of reflections from the rocks and water at the top and bottom of the falls.

Polarizer set for maximum effect

Polarizer set incorrectly

(Only post-processing was matching the color balance on the waterfall. Identical exposures (except zoom setting).)