Maybe you are an artist who created images and published them on the web. And then you find those images are used from other 3rd parties without letting you know? This is so-called “copyright infringement“.
Or maybe you are belonging to a company and want to know if your precious corporate identity graphics are used only by your own company and no one else?
Then you should really do an extensive reverse image search on those photos you own! Please use all of the methods listed here on this site, especially of course the Google Reverse Image Search feature, and also the TinEye Software.