Websites want to serve rich multimedia experiences. For an example, see the promotional site for the new Majora’s Mask 3D game. That site in particular isn’t sufficiently optimized (the images aren’t optimized and lazy-loaded - applying these techniques would reduce the page weight significantly), but still, a fair amount of images are needed to create the experience and on mobile devices high-resolution versions are needed, so the page weight can only be optimized so much.
Mobile networks usually have data caps, so a user may want to ensure that data is used conservatively when viewing websites. Ideally, a website should be able to serve a low-KB version on user request. Opera and Chrome have innovated in this area by providing a setting to dynamically compress page resources (using their own proxy servers), but maybe this feature would be useful in the web platform itself… somehow. At least, there could be a standard HTTP request header for signaling that a low-KB version of the web page should be served if available. This header could be activated via browser setting (just like DNT).
tl;dr Sometimes websites want to serve rich multimedia experiences that require lots of KB. Some users want to conserve KB. Do we need a standard for signaling to websites to serve a low-KB version on request?