Some loads were shipped by sealing the door opening with heavy paper, and then closing the door. Not paper grain doors, which were also used, but actual sheets of paper, covering the door opening. The edges of the paper would stick out on either side of the door. With roof hatches, that could be done here. The car could be unloaded by opening the door just a little, and tearing the paper.
It wouldn't work for grain, because the door had to be closed before the car was loaded, or the weight of the load would tear the paper. With regular grain doors, the grain could be loaded over the top of the grain door. "Grain" doors were also used for other loose materials which could be loaded the same way.