There is another purpose for this: long exposures with little noise. Or extend a long exposure to an ultra long one (can use just a 10 stop ND to achieve a ~4 or 5 min exposure in bright daylight rather than stacking NDs)
Basically shoot multiple long (or long enough) exposures (each image will be the correct exposure, unlike in astrophotography) and combine them into a single long exposure image. Combining the images/averaging them takes away the noise and also adds the motion from each long exposure.
In post, you would open all the images up as layers, then each layer would be (100/x)% opacity, so bottom layer will be 100%, second from bottom would be 50%, third from bottom would be 33% and so on. Tedious in post without some form of automation. With Photoshop I think there are image stacking tools, but in GIMP, you probably have to write your own script which I don't know how to.
Either way, the advantage of having this in-camera is that you can preview the final result, rather than waiting for you to get home to combine in post. If the results are not satisfactory, you can at least shoot another set of pictures, but if you do it in post, it's guesswork as to what the final image would look like.
