Ask HN: What's the use-case for this Responsive Web Design script?

The script: http://viewportindustries.com/blog/automatic-responsive-images-in-wordpress/

If I load a site on my phone the screen isn't going to magically become bigger. If I load it in a browser, even if it's small, there isn't a problem with loading the bigger image. As I understand, the problem with Responsive sites is loading big images in bandwidth-restricted browsers. Why the need for ajax? Am I missing something?