On another note, NDs are quite useful for advanced P&S e.g. KM A1/A2/A200 or Panasonic FZ-series. These cameras typically have a smallest aperture of only F8 and would be too large for some purposes, e.g. fireworks that may require say F13 at ISO100. So ND can be used to further "reduce" the aperture.
ND is to allowed larger aperture or longer shutter time to be used. ND is to decrease light entering lens.