The dam generates a low pressure area just behind itself which gives the air that has passed thru the radiator and engine compartment a place to exit. Removing the dam just forces the fan clutch to work harder because it now has to push the air thru the engine compartment. I would guess that certain air flow conditions around the truck would impact this even more like strong crosswinds and so on.
If you study the design a little on this vehicle (and most others) you will notice the front end kind of scoops and funnels air into the front of the truck across the heat exchangers (trans cooler, radiator et.) then it passes thru the engine compartment and must exit down and out the bottom of the truck. The dam causes a low pressure area which help evacuate this air which has now collected a lot of heat. While running at speed on the road all this air flow happens without much help. When you slow down or stop the fan must do all the work. Removing the dam just makes the truck think it is not moving and puts the load on the fan.