It is not uncommon for a photo to be retouched more than once. Maybe you spent a night sleeping over it, maybe you learned a new trick and most commonly: maybe the client came back with some feedback.
In this case, you will go back to the files and re-do some of the work you already did. If you did it all on the background layer, you may find yourself in a bad situation. The work that requires a fix is already used in another layer. Say a liquify filter. Since you can not un-liquify an image, you will have to redo everything and then liquify again. But this is true not just for liquify, it’s true for everything: healing brush, dodge and burn, working on skin, or on eyes or on lips. Fixing the skies. This is why we use layers.