Pallet session: What is the purpose of the `proof` argument on the `set_keys` extrinsic?

Question

The set_keys extrinsic in pallet_session has a second argument proof, which seems to be intended as an ownership proof of the keys.

Looking at the body, there is a call to ownership_proof_is_valid.

The ownership_proof_is_valid method is defined on the OpaqueKeys trait, with a default implementation that just returns true.

The intended way to implement OpaqueKeys is, as far as I know, to use the impl_opaque_keys macro. That macro, however generates an implementation that does not override the default implementation of ownership_proof_is_valid.

It seems to me, then, that in practice ownership_proof_is_valid will always return true. If that's the case, what is the purpose of the proof argument?