For wider apertures or longer exposures ND filter reduces the amount of light entering the lens so wider apertures can be selected, which is perfect for portraiture to reduce depth of field. Subject appears crisp and clear while the background becomes a soft blur. Also widely used for photographs of waterfalls and other nature scenes to emphasize movement.
ND4 filter reduces the shutter speed by 2-stops while ND8 reduces the speed by 3-stops. For example, instead of using 1/15 of a second shutter speed, you can use a much slower shutter speed of 1/4 of a second with a ND4 filter (1/2 of a second with a ND8). Slower shutter speed is ideal if you want to achieve the silky effect of a waterfall shot or to capture the light trails of road traffic at night.