Watt measure your energy output and is a measure of how hard you work. Watt measures your performance instantly whereas heart rate only shows how hard it feels for your body, and it has a delay of up to several minutes. Heart rate can be affected by other external factors such as room temperature, stress, time of day and other variables. Therefore, watt is a more precise metric. You can follow your progress by seeing how the watt you can output at a given heart rate change over time.