ND filters are a good way to have control over the exposure in your action camera image, particularly in bright light where you might want to bring the shutter speed to to the lowest possible and the ISO at a minimum. Why would we do this? Again, if we use the previous example of moving the camera from a shadow area to bright sunshine during the shot, if we don't fix the exposure you'll see a drastic shift during the shot. When the camera is in shadow, if the white balance is left to its own devices it will adjust the colour temperature to a higher setting, making any sunlight areas of the image that are in view very 'warm' looking.
For example, if you mount the camera onto a selfie stick or pole, you can start high in the air facing the opposite direction of where you want the shot to….