Updated: Feb 4
When VW changed their thermostatic control system, and moved from a ring that restricted flow into the fan shroud too flaps inside the fan shroud, those flaps actually served two purposes.
The first purpose, was to provide for rapid warm up. When the thermostat is closed, the flaps are closed, and restricts the airflow, and mostly restricts the airflow from the top of the cylinders and the fins of the cylinder heads. As the temperature under the cylinders increases, and the thermostat starts to open, those flaps open and start allowing more airflow. Once the thermostat fully opens, the flaps are aimed directly at the top of the cylinders, the cylinder head fins and the opening in the exhaust port sides of the heads. Because the airflow coming through the fan shroud swirls, the flaps serve a second purpose.
The second purpose is to straighten the airflow out of the shroud, since it swirls to some degree inside, and make sure that the majority of the airflow goes to the most critical areas of the engine, which are the very tops of the cylinders (they have the largest fins because this is where the majority of the heat that is not in the cylinder head is), the cylinder head fins, and the exhaust port side openings of the cylinder heads. The latter on the cylinder heads is the most important area, and isn't restricted by the flaps at all, even when the thermostat is completely closed.
With that technical background, if you build an engine without a thermostat, and without the thermostatically controlled flaps, you are actually not getting the best possible airflow to the most critically areas of the engine for cooling. Also, aftermarket fan shrouds, for the most part, don't even have the holes for the screws that are needed to install the flaps.
So, if you don't want to run the thermostat, or cannot because of your exhaust system, and you either have an aftermarket fan shroud, or just don't have flaps, you need this product to get the cooling system performing at it's optimum.