Try clicking between the original scan and the edited copy. Few pictures are perfectly exposed from the beginning, even fewer are perfectly printed and you very rarely see perfect scans. All this ads up to pictures, which have an inferior dispersion of light. This can mean several things, the most common being poor contrast - too much or too little. The second most common problem is color errors.
Many pictures are off balance, being too red, green or whatever.
The third most common problem is too light or dark pictures lacking tones in a whole region rendering them quite flat to look at. None of these errors are easy to adjust if they are very pronounced, but if they are within reason they can be all be removed. If you really want to understand picture adjustment, you want to understand tonal histograms or tonal ranges. A histogram graphs the number of pixels at each brightness level in an image. It is shown as a bar diagram with each vertical line representing the number of pixels at a certain level of light.