Neutral Density filters have four main uses

To enable slow shutter speeds to be used, especially with high speed films, to record movement in subjects such as waterfalls, clouds, or cars

To decrease depth of field by allowing wider apertures to be used, which helps separate subjects from their background

To decrease the effective ISO of high speed film (above ISO 400) and allow it to be used outdoors in bright situations

To allow cine and video cameras (which have fixed shutter speeds) to film subjects such as snow, sand or other bright scenes which could cause overexposure