When I mirror an image, either in Photoshop or Illustrator, it is usually to help fit another element into the design. It is extremely useful when adding other images or to help balance out text. It is also very simple to do and you can get the hang of it quickly.
I found an image today [from "http://magento-themes.joomlart.com/jm_mesolite/"] that has been altered and mirrored. It may be because the site graphics person is concerned about copy write infringements or perhaps it was over looked for another purpose.