Radiator caps for pressurised cooling systems

As of the past 20 years, a pressurised cooling system has become commonplace in modern vehicles. Modern engines generally use higher pressure cooling systems, which therefore require effective sealing.
A failure to do so would result in some pretty sticky situations such as:
inefficient cooling
loss of coolant
overheating
Pressurising the system raises the boiling point of coolant, so in extreme conditions, this creates a buffer by allowing it to run at higher temperatures.

Doing this prevents damage to the car, engine overheating and helps keep the system running smoothly. It also means that manufacturers can build engines with higher operating temperatures, which is especially common in performance vehicles. The radiator cap is a vital part of this process.