You also get additional protection because rather than each website holding onto a hashed (hopefully) copy of the user passwords that can be stolen in bulk, stealing the public keys for a passkey from a site wouldn't compromise the account. Someone would have to get access to your physical device or hack your password manager individually to get access to your passkey.

And and, the magic for most people is no more passwords and 2 factor stuff to deal with. The standard is still new, and in the cases where you want to use physical keys, its always best to keep 2 in case one gets smushed or goes through the washer. Some sites that have passkeys enabled only let you have 1 passkey. So in that case its kind of risky to make a passkey the only way to sign in.