You don’t have to count. Breathing hard after a run, sweat running in your eyes, your pulse pounding in your ears, it’s difficult to focus accurately on that throbbing little point on your wrist or on the side of your neck. A heart rate monitor allows you to measure at a glance, without interrupting your workout. You can avoid straining yourself. Don’t wait for your lungs to start burning!
If you’ve been working your heart too hard for the average person in your condition (age, weight, etc.), the monitor can tell you so. On the other hand, you can also tell if your workout wasn’t quite strenuous enough to limber up your heart.
Habitual walkers and runners build up cardiac “tolerance levels” for a certain amount of exercise over time. After a while, further effort is necessary to raise the heart rate sufficiently for a “true” workout. A heart rate monitor tells you just how hard you’ve been working, measured in beats per minute.