AtomPub uses "optimistic locking" as a way of managing conflicting updates to resources. For example, two clients may have run a GET at the same time, but once one PUTs/PATCHs, then the changes in the other might not be relevant anymore.
To help consumers know if they are working with the most up to date state, AtomPub returns an ETag on GET and consumers can add an If-Match with that ETag on PUT.
This is probably less of a problem on PATCH than it is with PUT (which we likely will not support), but it still seems like it could be useful for some use cases.