Sorry if this has been dealt with elsewhere but presumably a parcels train could be split and added to during its journey from A to B. So us modellers wanting to add a bit of interest to usual train comes into station, stops and then leaves again. So we could model these movements.
Train comes in, couple of vehicles removed from front or back and shunted into a bay. Perhaps a couple of vehicles added from the bay added to the back or front. Remarshalled train pulls out.
Later in the day parcels train heading somewhere else comes in, vehicles in the bay added, train pulls out again