This reports the actual operational (running) state of the BFD session as negotiated with its peer, which is the field that directly reflects whether fast-failure detection is actually functioning right now, as opposed to the admin state's mere intent. An admin monitors this because BFD's whole purpose is sub-second link/neighbor failure detection for routing protocols like OSPF or BGP, so an unexpected transition here can trigger a fast reroute before the routing protocol's own slower timers would have noticed anything. For example, cienaCesBfdSessionOperState flipping from 'enabled' to a down/failed value would explain why a BGP session over that link dropped almost instantly instead of waiting out its normal hold timer.