Remy sums up the psychological end goal of progressive apps (HTTPS + Service Worker + manifest JSON file) prompting an add to home screen action:
This high bar of entry will create a new mental model for our users.
If I add this app to my home screen, it will work when I open it.
It’s a shame that this charge to turbo-boast the perception of the web on mobile is a bit one-sided: I would love to see Apple follow Google’s lead here. But if Android succeed in their goal, then I think iOS will have to follow suit just to compete.