I guess you are considering the case where a single signer transfers commits over SSH to a single verifier, with both putting the same meaning into the act of t

I guess you are considering the case where a single signer transfers commits over SSH to a single verifier, with both putting the same meaning into the act of transfer as people normally would into a signature. But git is a distributed VCS, with signatures handling many more scenarios: consider multiple committers and multiple signatures, transfer over multiple, different, untrusted channels, multiple verifiers.