Without the guard heater, cooler air surrounding the edge of the main heater would be heated by conduction and convection. Thus some of the heat supplied to the main heater would be carried away by the surrounding air.
With the guard heater in place and adjusted to the same temperature as the main heater, the air in the gap between is maintained at the temperature of the main heater, so no heat is lost at the edge of the main heater. All heat lost from the main heater must flow into the test slabs.