What are these keys used for? Is it for 2FA? I would love to use my phone for 2FA, and not have to carry an usb device with me, because

- I often log in to web services on my phone, and I don't have an usb adapter cable with me to use the key, and

- because having to carry a device always with me sounds cumbersome. My phone is always with me anyways.

Is there some other use case I am overlooking? I am eagerly awaiting browser vendors to implement these use cases: https://w3c.github.io/webauthn/#use-cases