A Heart Rate Monitor can enable you to acheive the most from your aerobic training. To do this you need to use the Heart Rate Monitor to ensure that you are working within your optimum training zone (between 75-90% of your Maximum Heart Rate).
Using a Heart Rate Monitor you can use targeted heart-rate training to help your training. Training in a heart rate zone (percentage of your maximal heart rate) appropriate to your goals, the type and duration of activity and fitness level will greatly benefit your aerobic fitness.
A Heart Rate Monitor is like a rev counter, giving a precise measurement of exercise intensity immediately. Using a Heart Rate Monitor will help in your training (See Aerobic Fitness and Heart Rate training Guide) and will demonstrate how much fitter you are becoming as with increased fitness the Heart Rate Monitor will show that your heart rate is lowered as it is able to pump more blood with every beat.