For every pound of pressure exerted on the coolant in the system, the static boiling point of the coolant is raised by approximately 3° F.
Most liquids have a specific "boiling point", which is the temperature at which the liquid begins to change to a gas. If pressure is applied to the liquid, it must become hotter before it can boil. Pure water in a cooling system will boil (at sea level) at 212° F. At higher altitudes, atmospheric pressure is less than at sea level. Example: Water at 5,280 feet will boil at a mere 203° F. A cooling system that is under 15 pounds of pressure however, will now allow the water to reach nearly 250° F before it can boil. Even at this temperature the water is able to circulate through the engine, cooling parts that are at a much higher temperature without the water boiling. As long as the coolant remains in liquid form it can do it's job and transfer heat to the radiator so it can be dissipated. Coolant that is boiling cannot transfer as much heat and engine overheating is likely to occur if the coolant turns to a gaseous state. Steam adjacent to a hot surface, such as a combustion wall, can actually act as an insulator - thus preventing any heat transfer to the coolant.
Plus, modern antifreeze/coolants are designed to keep the boiling point of the radiator fluid mix at least to 250 deg. F. This is without any pressurization at all, so remember to keep your radiator mix at 50/50 even during the Summer.
This also helps with the lubricants, anti-rust, and anti-corrosion components of the antifreeze/coolant, it helps keep your vehicle HAPPY.