It serves to maintain proper system coolant level at predetermined pressures. It must always be replaced with an exact replacement cap with the same pressure setting. A belt-driven fan blade for pulling air through the radiator is usually on the water pump pulley and should have a fan clutch to control it. Both types are controlled via a temperature sensor - in the radiator or upper radiator hose or on the thermostat or water pump housing. Your car may have other sensors as well.
Temperature control is critical to both performance and emission control.
Unfortunately, this system is the most difficult to troubleshoot without proper equipment and diagrams. It's even more difficult with computers that adjust timing, idle speed, vacuum and fuel delivery automatically to make up for potentially faulty temperature sensor signals. Checking and replacing all parts at the factory-recommended time or mileage limits helps as well. The cooling system is designed to function with all other systems operating properly. Come back often to check for new maintenance topics.