“Save a Copy” does the same things as the Save command, with one important difference. It always asks for a file name, saves the image into the specified file, but does neither change the name of the active image nor marks it as "clean". The effect of this is that if you try to delete the image, or exit from GIMP, you will be informed that the image is dirty and given an opportunity to save it.

This function is useful, when you want to save a copy of your image in its current state, but continue to work with the original file without an interruption.