Circular polarizer helps photographer to reduce the reflections and limit glare. Especially on nature photography; the circular polarizer is useful.
For example, its possible to remove or control the amount of reflection on water or shiny surfaces, Darken the sky and bring out details on clouds bright, sunny days. This is done by rotating the filter itself.
These two photos are taken in a row on my last trip to Norway. It was quite windy evening and overall lighting changed a bit during long exposures but you can clearly see how the polarizer works. Filter is rotated 180 degrees.
Gear used: Canon 5DMk4, Canon EF 16-35 f/4 L IS
Haida 100Pro-filter holder, Haida 100Pro 82mm circular polarizer
Photo: 16mm, 8s, f/11, ISO100