Polarizing filters allow you to increase the video footage quality with of your dash cams by reducing glare and reflections on the windshield and increasing the color depth and quality of the videos.
These filters really excel when the outdoor lighting conditions are likely to lead to poor video quality. An example of such conditions would is when the sun is directly overhead, causing reflections of your dashboard on the inside of the windshield. Another example is when there is a lot of glare when driving directly into the sun.
The images below show images from a dash cam without the filter, and with the filter, and really depict the huge difference polarizing filters make.