Air in the heater core will allow partial evaporation of water out of the coolant. Over time, this very corrosive, concentrated coolant will eat its way through the aluminum where there is no solid stream of coolant flowing and perforate the heater core. Keeping your system topped up and air-free keeps you from having blown out heater cores or eaten out and leaky top passages in your radiator too. Keep the system totally topped up and air free as much as possible.