POB or souls on board is so RFF can determine if everyone is off or not by a head count. That way they don't have to send people into harms way unnecessarily. Regarding the fuel, I was told the RFF use the fuel on board to determine how much fire fighting agent they will need to deploy to the aircraft. I imagine giving ATC your endurance will enable them to help monitor the situation in order to avoid the aircraft running out of fuel while the crew are concentrating on a non-normal situation, as has happened in the past.