Please explain the interruption.
The fan being locked to the engine RPM prevents it from spinning at the RPM the air flow would otherwise cause. The physical term is turbulent air flow. Imagine holding up a fan spinning at the equivalent of 20 MPH when the head wind is 30 MPH.
Or a plane or boat prop. When one of those spins fast it creates a wake, which is the medium being forced to accelerate, moving the craft forward. If you disengage a hypothetical clutch as the boat is drifting the prop will just turn slowly. The pressure on each side of the fan is equal. If you hold it still or try to spin faster than air speed there's a pressure differential created, it might slow you down or speed you up but either way it's disturbing the air or water.
Trying not to get too technical but air flow in this case is relative to pressure, flow rate (cubic feet per minute, for example) and speed (RPM). The fan blades have a surface area that creates the flow necessary to move enough air over the radiator to meet cooling demand.
We talk a lot about these same things electrically. Pressure is voltage, flow rate is current, the two are related by resistance. If pressure is the same then for a given flow the resistance is in essence zero, which is what you want on the highway when just driving down the road is pushing plenty of air through the radiator. You don't need a fan and free wheeling saves energy and doesn't "resist" air flow.
It's the original reason for a fan clutch in the first place. Way, way back I know you remember and see on the Mercury. When the fan was hard connected to the engine it was forced to spin with the engine RPM.
But without a shroud that was OK, the air could flow from the edges. it wasn't optimal cooling but the radiator oversized and lots of surface, it wasn't
too much of a problem. As things improved the clutch became necessary.
The fluid in the clutch is temperature sensitive. When it gets warm it thickens and the fan spins up with the engine. When it cools it thins and lets the fan just turn at the speed of air going through the radiator. So on the highway the fan is usually just spinning proportional to the air flowing through, which is a different RPM than the engine most of the time.
You usually notice it overheating in traffic first because the nature of them is the fluid leaked and it no longer engages but the opposite is possible when it becomes contaminated and never thins, too.
Oh, yeah, then electric fans came in. Those only turn on in traffic, right? On the highway they're just spinning freely. You can limp home with a dead electric fan by just never stopping for too long.