Yes, I know.
My point is slightly different. At the moment I save a book for reviewing the changes outside e.g. ADE, I need first to save the file and overwrite then the original EPUB in the library. This is something what I like to prevent as long as possible, especially when I cannot finish a project within one session, what is quite normal for me. I am using calibre often as "nerve centre" with an own database for a project when I create or modify a book and its different documents and more often the process takes several days/weeks (what ever). So said, not quite the normal case using calibre, but sometimes very handy for me.
For sure, I can do this workflow outside the library or rather with manual backup copies (what I do), but I thought it will have maybe a benefit to do the whole process within calibre.
That is the story behind this request.