The use case they're trying to protect against is malware authors "coaching" users to install their app.
In November, they specifically called out anonymous malware apps with the permission to intercept text messages and phone calls (circumventing two-factor authentication). https://android-developers.googleblog.com/2025/11/android-de...
After today's announced policy goes into effect, it will be easier to coach users to install a Progressive Web App ("Installable Web Apps") than it will be to coach users to sideload a native Android app, even if the Android app has no permissions to do anything more than what an Installable Web App can do: make basic HTTPS requests and store some app-local data. (99% of apps need no more permissions than that!)
I think Google believes it should be easy to install a web app. It should be just as easy to sideload a native app with limited permissions. But it should be very hard/expensive for a malware author to anonymously distribute an app with the permission to intercept texts and calls.