I think the "Crew on stations" call is intended to cover all accident/incident scenarios and lets the cc know that the fd are alive, thinking and doing their job.
In the absence of any call it is not unreasonable, in many scenarios, for the cc to initiate an evacuation.
This call should prevent those premature or unwanted decisions, but leave the cc on alert until stood down or an evacuation is ordered. Either way if the "On Stations" call is made they know that one or the other is coming, and the fd are not incapacitated.