It stops it from running too cool because it acts as a small restriction. Without that, there would be too much flow and the engine would run too cool.
Here is the main ball of wax with all of this. Unless you have a radiator that is capable or more than capable of shedding the heat generated by your given combination, you will very likely be doing other "incorrect" things to get the engine to run cool. This one reason in itself is why most people have difficulty. They simply do not choose a radiator with enough heat shedding capability.