If the containers and packages were to be dropped from the bomb bay, then why the dispatcher was still needed?
The Air Despatchers were responsible for preparing, loading and despatching supplies and equipment from military aircraft.
The Dispatchers helped the agents or goods out of the aircraft .
and ...
There were other precautions: the targets for the drops would only be known to the pilot and navigator, although the dispatcher would be told the number and sex of agents they would be taking and the contents of any containers.
According to the two quotations , not all containers and packages could be dropped from the bomb bay. What is more the members of the crew didn't know all about a mission. Each one knew a part only. As a result the pilot knew the target/targets, the navigator the way/destination point/points and the dispatcher knew about what should be dropped there. IMHO that's the reason for the dispatcher's presence there.
BTW ... the dispatcher also could be the flight engineer or the top gunner.