Until you're a professional designer and they ask you to "scale up this part" or something. Then you realize that your actual intention is having the original layer currently rendered at that size and did not actually intend to permanently modify the original so you can't go back.

Once you export it is, of course, actually downscaled.

Same idea behind masks and modifiers and the like. Most modern creation tools try to give ample opportunity for non-destructive modification to simpler, immutable primitive layers.