When you need the person in the photo looking in the other direction, or you need the top of the image to be the bottom. Mirroring the image (sort of). Use Tools → Transform Tools → Flip, or use the button on the toolbox.

Using another APOD image [APOD07] I demonstrated all of the flips on this image.

The tool used as is (the default) will simply flip the image at its vertical axis. If you double click on the button,the tool options dialog will open, if it is not already displayed in the dock under the toolbox. Use the options dialog to switch between horizontal and vertical. You can also use the Ctrl key to switch between horizontal and vertical. All possible flips are displayed in this page.