> And if it's just a web site, why do you need to "install" it? A link is surely sufficient?

The point is that there are apps that can pretty much be built entirely using web technologies as PWAs, but in doing so they are no longer "just web sites" and they need functionality of installed apps, like notifications. For example, most banking apps on Android could be entirely rewritten as PWAs, but they'd need to make use of things like notifications (I don't want a random website sending me notifications, but I DO want notifications of activity on my bank account) and camera APIs (e.g. for mobile check deposit).