It was basically built so password manager extensions could communicate with password manager apps to do things like yubikey support etc. before browsers started handling webauthn
Yes, I have done this for (a fraction of) WebBluetooth. Albeit the extension API does not make it easy, and i don't know if you can implement the entire Web* API surface, but at least it is good enough for proofs of concept.
Extension + a native program for the extension to talk to. The native program has to manage accessing the usb device and providing an interface to the browser extension.
It was basically built so password manager extensions could communicate with password manager apps to do things like yubikey support etc. before browsers started handling webauthn