Most apps would do fine without a website. Most everyone has a phone, but a fair number of people don’t have a desktop or laptop. And pretty much everyone who has a desktop or laptop also has a phone.
A number of currently popular apps don’t have a website, let alone a mobile friendly website.
You’re missing the main point: A web app works for both desktop and phone users. A mobile app doesn’t.
And I, for one, don’t have a smart phone, though I’ll admit I’m lonely in this position.
A webapp sometimes works fine for phone users. There are things that websites can’t do on mobile. For example, on iOS, only the latest OS version has support for push notifications from PWAs, and even then, they can’t make noise or vibrations. They are always delivered silently. PWAs are also always rendered with WebKit on iOS. WebKit doesn’t support a number of features.
Yes, a mobile app doesn’t help desktop users, but there are waaaaaaaay more mobile users than desktop users.
on iOS, only the latest OS version has support for push notifications
they can’t make noise or vibrations
