If the coolant system is leaking water or air, the water will boil more easily.
The water coolant system in a car is designed to be under a certain amount of pressure. The pressure increases the boiling point of water so that it can safely reach temperatures above 100C without boiling. It's not much pressure - usually around 10-15 PSI, but its enough to allow the system to operate at a high temperature without boiling.
You may be familiar with water's boiling point changing at high altitudes. This is because of the change in pressure at higher altitude, which is the same concept as the pressurized coolant system.
This is all to say that if there is any leak at all, the system will not hold pressure, and the boiling point of the water/coolant mixture will be low enough to boil at normal operating temperatures. The boiling creates bubbles which lead to more heating problems and the result is usually a pressure that is high enough to force water out of the system or break something.