Until you need to log in on some random device. Or inside an in-app browser popup.
So passkey is saved to my account. Account is logged in on multiple devices, secured with an additional pin.
It doesn't matter what device I'm on, I can either use the app on a mobile device or a browser.
Site goes to login, prompts me for passkey, I type in PIN and select the passkey from my password manager.
It's fast and easy to use.
If I am on a random device (which never happens, ever) I would just log in via browser, or use one of my hardware tokens if I were expecting to access something from an unusual device.