My original reasoning was as a parallel to the Cookie class, which can parse itself from a string. (CC then turned out to be much more complicated...)

The obvious use case is if you were building any kind of proxy. Even if WM is intended as a source server, there's cases where you'd want to transparently check with e.g. github, and at that point you'd want to respect client cache requests. (CC is a single header for both sides of the exchange.)