non-repudiation
Being able to prove to someone else who did something.
Non-repudiation is the property that the originator of a message or action can be proven to a third party; signatures give it, shared-key MACs don't.
Examples
A command signed with an operator's private key can be shown to an auditor as proof that operator sent it.
An HMAC tag gives no non-repudiation, because the device and the server both hold the key and either could have made it.