>What would I gain from using a passkey?

The number one gain I see with passkeys is that they are phishing resistant. The biggest issue I've had with TOTP keys is they aren't phishing resistant.

Forcing people to use passkeys gives the authentication layer a stronger means to ensure that people are logging into the correct domain. A password manager accomplishes this, but it's very difficult to enforce. (I've heard a number of variants of "John Smith sent me an important document and when I went to open it, the password manager didn't work so I just copied password")