They have a similar effect on the image as Image Averaging.
For completion sake I also have to mention star trackers here. They have a similar effect on the image as Image Averaging. But instead of taking multiple short exposures to average them later, a star tracker allows you to take one long exposure at low ISO while maintaining pin point stars in the frame. Same as Image Averaging does, the foreground will also be blurred but the sky and the stars will look clean.