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Just imagine you've got a couple of whopping dust spots on your camera lens, then you take a shot. The image is compromised where the dust/gunk is. If you move your camera a little in any direction then snap again, then continue moving and snapping........then later when you combine the images the stacking algorithm will discard the "damaged" areas that should now be in a mirnority of frames.
The downside is you lose some of your frame size when you crop the final overlapping montage of multiple images.
On the other hand, if you kept tracking perfectly at the one spot all night, snapping away, you'd never "see" well what was behind the dust/gunk.
Hot and cold pixels can also be similarly dithered out, not just large scale dust. Hope that helps.
Then, there is drizzle.....
Rob
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