[ChrisMarshallNY]

The most obvious use, is to have a server that only delivers content relevant to the end-user device.

This would be very useful in limited-bandwidth scenarios.

Responsive sites aren’t the same. They deliver all the data, but filter it in the UI.

[yjftsjthsd-h]

Obviously the formatting differ, but why would you deliver different content to different devices?

[pavel_lishin]

Hypothetical example: When I open Twitter in the browser, I see a feed - but I also see a "What's Happening" section, and a "Who To Follow" list of suggestions, as well as what looks to be my inbox, minimized. Plus, the feed itself automatically loads the images that people are tweeting.

If you know a client is likely to be from a place where bandwidth is expensive, you may choose not to load the "What's Happening"/"Who To Follow", or the messages, or possibly even the image URLs (which I'd guess come from the backend with an array of URLs of those images in various sizes & resolutions.)

Hell, you might even load a smaller subset of the feed - 10 items instead of 30.

[ChrisMarshallNY]

1) End device has ability to display HiDPI images -> Send big

2) End device does not have ability to display HiDPI images -> Send small

Of course, if you have (1), in a low-bandwidth environment, then you actually want the server to send small, even if the device can handle big, but that can be indicated with a different flag.

[Hackbraten]

The `img` tag in HTML already supports that.

[ChrisMarshallNY]

Yes. But the same principle applies to things other than images. For example, regular PDFs, vs. optimized ones.