Even though more and more people get access to the internet every day, not all of them have fast gigabit connections or unlimited data. Using the media query prefers-reduced-data we can keep our sites accessible to everyone.
I’ve long wondered if there could be a way to tailor content to a user based on the speed of their internet connection, especially when considering features like responsive images. This looks like it might be the way to do that (although browser support is currently non-existent). It also allows us to respect the user’s wishes on how much data / battery life etc they’re willing to spare for your website.