One of the Google I/O talks mentioned that this feature (currently behind a feature flag) [[ https://youtu.be/Mv-l3-tJgGk?t=27m16s | is probably coming to Canary in a few weeks ]]:
I think it might be particularly useful for priorities between JS files and images. I.e. the article images near the top (eg. first 3 in-article images) could be given a high priority, higher than async JS, and the rest of the images given a low priority. And between JS files, we could also have different priorities.
Chromium discussion: https://groups.google.com/a/chromium.org/forum/#!msg/blink-dev/65lfM2f0eeM/-1ttGbZmCQAJ
And code review: https://groups.google.com/a/chromium.org/forum/#!topic/blink-reviews/8NB_WWooCFs