Screen Stretching is used to manipulate the overall dark and light contrast of an image. The bright areas might be toned down to allow details to show that would otherwise appear washed out. Changing the contrast range of an image can help emphasize the details you want to bring out. Stacking is a method of laying multiple images on top of each other and combining them into a single image. Much of the noise is random in nature.
Less noise makes for a smoother image.
It also allows for better filter processing. Filters used to sharpen an image tend to amplify the noise in an image as well. Some users may stack over 50 images. Stacking does have a downside, as it can lead to a loss of detail. There are also a number of unique filter algorithms used especially for astronomy, Maximum Entropy , Lucy Richardson , and Digital Development to name a few.