This is what I get asked by my friends all the time: It actually sounds pretty crazy without knowing the context of all this.
The answer: We stack all these sub-exposures for a better overall image. Stacking hundreds of the same image can lead to:
1) Increased dynamic range and more faint details showing up in the final image.
Single frame with 3 minutes of exposure (before stacking)
45 images of the same target with the same sub-exposure length (after stacking)
NOTE: no special processing was done to each image; they were just lightly stretched!
2) Natural noise reduction in the final image
Most of the time, noise kills astrophotos and it can be so bad that it renders some images unuseable
Noise profile of a single frame (Before stacking)
Noise profile of 45 images stacked in DeepSkyStacker (After stacking)
So you can immediately see the difference between a single exposure versus multiple exposures stacked in a software. For all of my images, I use Deep Sky Stacker as it's a free-for-all software that packs tons of features in the program.
Clear Skies everyone!