Heart Rate - The Window Inside the Body
Monitoring your heart rate is a form of biofeedback data - you know what is going on inside your body.
Although there are many subjective clues as to how your body is doing during exercise (perceived exertion, breathing rate, physical sensations), none is as reliable as measuring heart rate. It is objective and is affected by both internal and external factors - meaning you will have a dependable measure of your physical state.
A heart rate monitor gives you a physiological window, through accurate heart rate measurement, into your body's response to the moment-to-moment changes in your physical activity.