Most people think of using polarizers to darken a sky or remove reflections from water. They think of using them on bright and sunny days. Most nature photographers I know, know better. Sure we still use them on bright sunny days; but usually in our sunglasses, because bright sunny days often offer some of the worst light for nature images.
So when should you use a polarizer? And "all the time" is not the correct answer.
I use polarizers a lot on overcast or rainy days in the forest. There's a whole lot of light reflecting off of green leaves and needles and fronds. Using a polarizer will cut the glare coming off those leaves and needles and fronds and allow the true green colors to come through.
You can see this happening right in the viewfinder and the trick is to pay attention to the leaves while your turning the polarizer.
In the pair below, the one on the left is without the polarizer. The one on the right is with the polarizer.
Grab the slider and move it left and right to see the difference.