Because the tiny sensor on the Q doesn't like apertures below f8, in fact you can't go below f8 on the Q lenses, there are times when the light is just too bright, so they fitted a built in ND filter that you have to activate in the menu.
Other uses are to deliberately get slower shutter speeds, when photographing flowing water for instance, to get some deliberate blur into the shot. and to use wide apertures, for less depth of field for example, in bright light when you might not have a fast enough shutter speed available to let you use it.
Normally it's turned off.
It will make no difference to image quality, and has caught me out once. Low light, bracketed shots, on a tripod, it took me a while to realize why I was getting shutter speeds of several seconds !! Here's the image it took. 3 frames 2 stops apart blended in Photomatix Pro. I shoot a lot of HDR with my Q as it's so easy to get the bracketed shots, a huge plus for me.
Sheffield_04 | Flickr - Photo Sharing!
Chris