Since I already use a phone capable of doing the same thing, let my phone be my main authenticator, and then I can use a Yubikey as a backup.
It's not like one is necessarily better than the other, except that you already carry a phone and they're capable of being a hardware device that works with Webauthn. No need to carry a second device or, pay for one, for that matter. Since at least with Apple's solution it'll sync over iCloud Keychain.
If you're happy with Yubikey's, nothing changes. But for the average person, this makes Webauthn an option without having to buy any hardware or carry something you are more likely to lose because you don't understand the intricate details of how the thing works. I wouldn't expect my parents to understand how a Yubikey works well enough to know it should be used as a pair, for backup purposes, but that is a barrier to entry for them that they don't need to worry about now.
Once passkey support comes to bitwarden I'll be a little more comfortable I think.
Thus, unlike a FIDO2 key, you don't have to visit every online service to tell it about the new redundant keys you add.
The rest of the security article linked by madjam002 goes into detail how Google implements their version of that backup. It's a bit like Keybase in the sense that your other devices act as keys to unlock the backup for new devices.
Yubikeys are great but they're super niche. Among Android users alone there might be a billion people who will never buy one.