What is the purpose of a pressure cap?

Answer 1

A pressure cap is used to determine the pressure in your radiator. It raises the boiling point of the coolant. If the coolant boiled, it would cause gas bubbles to form and cause hot spots in the engine cooling chambers. The pressure valve has a spring set on a certain PSI (dependent upon the vehicle, I suppose) so that when/if the pressure in the radiator gets too high, the pressure is enough to force the valve temporarily out of the way, relieving the excess.

Most modern cars have an expansion tank at the top of the system. The coolant, bypassed by the pressure cap, ends up temporarily in the plastic bottle and is then drawn back into the system as the engine cools.