The cooling system is pressurised to raise the boiling point of the coolant. This is to prevent the evaporating coolant cause air to form in the circuit. A car engine is designed to run at a desirable temperature. To maintain this the system must be designed to reach equilibrium within an acceptable controllable range. Hence a radiator’s heat exchange capability is set to do this, in conjunction with airflow through it. Add in other complexities of of thermostats to manage warming up, temperature controlled fans to cope with extra heat on some occasions ( stuck in traffic, extended higher speeds) coolant volume materials used in the engine, and systems etc . The use of a pressure cap means this boiling point is raised. Most cars use 15psi caps but others might not.
A pressure cooker works using this BP elevation to increase the temperature to speed up cooking. Likewise, mountainous areas that are at lower air pressure due to altitude have a lower BP for cooking in water. A liquid boils when its vapour pressure equals that of the atmospheric vapour pressure. At altitude the vapour pressure is lower so water boils at a lower temp.
In the opposite direction, an antifreeze solution freezes at lower temperature than pure water.
It’s possible to calculate both effects fairly simply but I forget the formulae. I left education many years ago but the physics, chemistry, fuel tech was fun (modtly!)
