It's also used to enable serving cross-origin web fonts.
In the case of the author, he wants to use Flickr images in an HTML5 canvas element. I would set up a server-side proxy for that (didn't see this mentioned either).
If you host on a service like Heroku, server-side proxying ties up a dyno needlessly. If you can get the host to enable CORs, you can save a lot of hosting money. I'm not sure if it's as worthwhile for self-hosted/VPS setups.