My belief is that the panel at the top endures that air flow from below is more completely directed up and over the cooling fins. Without it, the flow would be less forceful at the upper end of the large opening, where the fins are located.
I think the two fan solutions are actually for two different problems. Exterior fans improve the gross functioning of the cooling system. Internal fans improve the effect of that cooling as it applies to refrigerator contents at all locations inside. They compensate for airflow obstruction caused by that big milk jug, the crammed-in Tupperware, etc.
2019 K-2500 Duramax Crew Cab
2014 Silverback 33RL