Why is so important to use adjusting layers in Photoshop?

The original image is equivalent to the negative in film photography. I think nobody would taken a negative, scratched it and cut it up in pieces to see if they liked the printed image.

Working with layers gives us the opportunity to have an absolute control over the whole adjustment process, and it preserves the original image as a reference.

Every single step we take, we should do it in an individual layer, if we need to cover minor imperfections, we can have a layer where we clone or use the healing brushes.

We also should have individual layers to apply corrections in color, contrast, saturation, etc. and use the layer masks to have a perfect control.