The History palette shows you every step you’ve taken since you opened your picture. At the top you see that I’ve opened the picture of Hunter. I changed the brightness/contrast and vibrance, then I changed the image size and saved it as a new file.
If I decide against changing the image size, I just take one step back to Vibrance. Often I’ll make a few changes to a picture, then go back several steps to see if the changes I made were better or worse than the picture I had before. It isn’t infinite, so be aware that you can only go back in time so far. Why can’t this be real life?