Keeping the pressure up on the coolant also increases the heat transfer rate between the cylinder walls and the coolant. Otherwise small bubbles would form on the hot metal internal surfaces, then (over time) displace coolant from the system.
Those bubbles are the same concept as boiling a kettle or billy on a fire or gas ring - as the metal kettle/billy etc starts to get warm, small air bubbles form where the heat transfer is taking place. With pressure on the system (ie; a functioning radiator cap) the formation of those air bubbles is (almost) eliminated. I've set my ute up with a 16psi recovery set up for the last 17 years without an issue apart from blowing a welch plug early on when I first set it up. That extra one psi (factory aircon car originally had a 15 psi non-recovery cap) was enough to finish that 25 year old plug off.
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