From the world of client hints comes Save-Data:
The token is a signal indicating explicit user opt-in into a reduced data usage mode on the client, and when communicated to origins allows them to deliver alternate content honoring such preference – e.g. smaller image and video resources, alternate markup, and so on.
The Google Developer site has a write up on Delivering Fast and Light Applications with Save-Data. No surprise that their main target is a better experience for users on slow mobile connections.
If sites start respecting Save-Data = "on" requests, and they do it in a way that maintains basically functionality, then why not turn it on all the time? Even if I’m on an LTE connection, I’d rather have a faster loading site. The next step is to turn it on for desktop browsers. There is nothing that limits this to mobile clients.
Anything that targets faster page loads on mobile clients will eventually get used everywhere.