With a standard thermostat, once the temerature in a room has been reached the system shuts down, however heat still remains in the pipes, radiators etc and will continue to heat the room for some time, raising the temperature above the desired temperature and wasting energy. When the temperature in the room drops below the desired temperature the heating is switched on however there is a delay while the hot water makes its way round the system, again wasting energy. If the actual temerature in the room was to be plotted on a graph, there would be peaks and troughs above and below the desires temperature, each of these indicating wasted energy.
With a heating control system this problem can be avoided easily as the system will not just wait for the temperature to fall below a threshold but will continuously monitor room temperatures and adjust the heating up and dowm accordingly to maintain a steady temperature, eliminating peaks and troughs and wasted energy.