Neutral Density Filters reduce the amount of light entering the camera, enabling a longer exposure time than otherwise possible. On the GoPro camera this means reducing image "jello" and "jitter" since it decreases the GoPro shutter speed. If you are shooting in ultra bright lighting, using an ND filter will help bring the light levels back to prevent highlights from blowing out. ND filters can also help emphasize motion, or make an otherwise tumultuous scene appear surreal and quiescent. They will also enables larger apertures which can produce a shallower depth of field, or achieve a sharper photo.