If I'm understand correctly, the resident keys can be used in the case of a non-ECDH scheme and otherwise they wouldn't be used? How flexible is the FIDO2 specification on crypto schemes?
WebAuthn adds a number of crypto schemes -- to wit, I think they add RSA. You can certainly deterministically generate RSA keys but it's a lot more of a pain in the neck than x = HMAC(k, "u2f" + custom); P = xG :)
It is now starting to make sense to me why. As jiveturkey pointed out, it allows usernames to be stored. And, as you're pointing out, it's useful for RSA or maybe other crypto. Thanks.