I'm just curious — what could be a potential use case for such things on the backend? For bot detection, it seems quite unreliable. Would it be more suitable for server-side rendered UIs? Or am I missing something?
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.
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.
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.