One of the popular usage is in affiliate programs, where you 'build' your own shop while it leans on the original store images.

TheRaveN 2.0

I have noticed this phenomenon on my blog, a great deal of traffic comes from Google Image search results, for images that I do not host.
I am getting these results from Feedburner, which doesn't seem to recognise all these result pages as Google Image search results, only some of them, but it seems the traffic comes mostly from a local variant of the GI search (though those coming from the .com domain might be the ones actually recognised, for which I can't see the actual query words).

Images are a fantastic way to boost traffic. Learning how to optimize image searches by always linking each image will dramatically boost your website.

[Link removed – Tony]