My understanding is that it was designed for other third party browsers to adopt and use as well as it's the better and more up to date web view controller/container than the previously existing one (and one that still exists) that the third party browsers use.You're right it doesn't affect safari but it's not really designed for other browsers to use. It's designed for other apps to briefly display web content. Eg you click a link in Facebook and rather than take you out of the app to Safari, Facebook built a simple browser with a web view and some controls. Now Apple want people to use the safari controller. Similar in that it doesn't take you out of the app but now it's more familiar and you get auto fill etc.
Now whether creating an app which is solely this safari view is against the rules I don't know, but it's certainly not what it was designed for
I did actually have this idea a few weeks ago but lack gumption 😀