1. SSH keys, as they're normally used, let you be tracked between hosts. That's fine for SSH, because nobody's trying to SSH into their Grindr account. But for web login stuff you want a different key pair for every site.
2. Adds a bunch of 'attestation' features that corporate types think they need.
3. Tries to make it so an attacker who gets access to your machine can't make a copy of the credential. The success of this is implementation-dependent.
4. With barely any setup, Google/Microsoft/Apple will keep a backup copy, in case you lose your phone. This is useful for non-technical people.
