If a coolant boils, it can be as serious for an engine as having it freeze.
Boiling coolant in the waterjacket becomes a vapor. No liquid is left in contact with the cylinder walls or head. Heat transfer by conduction stops. Heat builds up,
And that can cause serious damage.
One way to prevent this is with a radiator-pressure cap that uses pressure to change the temperature at which water boils.
As coolant temperature rises, the coolant expands and pressure in the radiator rises, and that lifts the boiling point of the water.
Engine temperature keeps rising, and the coolant expands further. Pressure builds against a spring-loaded valve in the radiator cap until at a preset pressure, the valve opens.
In a recovery system, the hot coolant flows out into an overflow container.
As the engine cools, coolant contracts and pressure in the radiator drops. Atmospheric pressure in the overflow container then opens a second valve, a vacuum vent valve, and overflow coolant flows back into the radiator.
This system stops low pressure developing in the radiator, and that stops atmospheric pressure collapsing the radiator hoses.