I recently came across a website, jackis.com, that did a nice job of highlighting three good reasons to track heart rate: safety, effectiveness and incentive. I would suggest one more accuracy. If you want accurate an accurate calculation of calories burned during your workout, it needs to be derived from your heart rate.
From jackis.com: One of the goals of your aerobic workout is to improve your cardiovascular fitness. Heart rates taken during exercise indicate how hard your heart is working. Your heart rate is actually a motivating friend when you learn to monitor it properly, for this allows you to objectively detect beneficial changes that you can’t otherwise see.
The benefits of monitoring your heart rate are:
Safety. The heart rate is a gauge by which to assess the intensity of your workout to make sure you’re not overexerting or overextending yourself. For example, if your heart rate is above your working heart rate range, it’s telling you to slow down a little and use fewer arm movements.
Effectiveness. If your heart rate indicates you’re not working hard enough, then you can work out a little more vigorously to maximize the effectiveness of your workout. To maximize your aerobic workout, you need to stay in your working heart rate range for at least 20 to 30 minutes continuously.
Incentive. By monitoring your heart rate from week to week as you participate in an aerobic activity, you’ll discover that you will be able to exercise at a higher level of intensity, but at the same or lower heart rate. This is the way the heart tells you it is becoming stronger and more efficient. When you see positive results, it will motivate you to strive for even better results.