Many years ago engine water pumps ran full time, pumping the water through the engine block but not through the radiator. Thermostats in the top of the radiator contained a spring-loaded valve, the spring being made of two dissimilar metals (bi-metallic, just like the ones in your home) and when the temperature got to 180o, the metals would expand, open up the valve, and permit the water pump to start pumping the water though the radiator where the onrushing air due to the vehicle's motion would cool the water and keep its temperate at 180o. That means the coolant water in the radiator was not boiling, and it was safe to take off the cap. The radiator, if vented to the atmosphere, was prevented from building up any internal pressure because no boiling took place.

Imagine this experiment
Take a glass of boiling water and set it on the sink top when the house is at 70o temperature. Time how long it takes to cool down.

Now put the same glass of boiling water in the freezer and it will cool down much faster. Why?

Because the temperature difference was greater.

The rate of cooling is directly proportional to the difference in the two temperatures
Noting this phenomenon in Nature, auto makers concluded that a much hotter radiator would get rid of heat faster and therefore could be made smaller and cost less to make. A smaller radiator would also reduce the car's front end profile, making it more streamlined and esthetically pleasing.

To do so, it had to seal the radiator.........with a better cap.....and make a better radiator to hold a little pressure.

Modern engines and radiators
Currently car radiators operate at about 210o to 220o, right about boiling water temperature. But remember that if one boils water in a container where the pressure is greater than atmospheric, the boiling point will be higher than the familiar 212o.

Therefore by making the radiator able to withstand higher pressures, the coolant can go up to 220o and transfer heat faster and be smaller than Grampa's 1945 Buick with its180o thermostat. An added bonus is that the hot water radiators, in the cabin of the car, used to heat the passengers, can also be smaller and cost less because they will be processing hotter water than Grampa's 1945 Buick.

The radiator in a modern engine must be tightly sealed
If the cap on a hot modern radiator is removed, then all the water in the radiator, which is, for example, at 220o, above the 212o boiling point at sea level, would immediately turn to steam. But when water turns to steam it expands about 1,700 times, which means it will likely remove the hood of the car in one terrific explosion and scald the skin off the face or hands of the person who just unscrewed the cap.

If a hot water boiler in a basement of a house has a failed thermostat, and the water goes over 212o before the tank ruptures, the resulting expansion of 30 or 40 gallons of water into steam equal to 1,700 times the volume of the water will turn the house into a pile of twigs. And it has happened more than once.

So, never, ever, remove the cap on a hot radiator in a modern car. And always keep the makeup reservoir, usually mounted under the hood against one of the fenders, at least half full.