Besides, every time you make a change you must comment why you do it or what kind of change it has. It can be used to keep a writing diary that will, later on, help you with editing or with finding out why the hell that character suddenly disappeared in chapter 5. People following your work (who knows, there might be somebody interested or maybe just your editor or your mother if you are an Indie writer) can comment on the change and voice their opinion. Which leads me to reason number.