(How does cropping figure into all this? Almost every one of my uploads is cropped, sometimes by quite a lot. Does anybody really want multiple megabytes of out-of-focus background shrubbery? :-) )
Sometimes you want to adjust the aspect ratio of an image (say to make it fill the width of a wide taxobox without taking up too much height). If you don't have an orginal with plenty of background to work on this is much harder (since you would have to create more background or lift the foreground image onto a new background).