Heart rate monitors

Calorie tracking? Gotta refuel, ya know.

In addition to what has been mentioned about training (a good book on HR training and zones, the benefits and such of training in each zone, and free to borrow on a kindle: http://www.amazon.com/The-Book-Endurance-Training-Racing/dp/1616080655) and fueling, it also helps with telling whether you're overtraining or getting sick.  Since I've switched to using my HR monitor all the time and staying in the proper zones, my training has gotten more consistent. I, like many other triathletes who tend to be go getters, tended to overtrain and paying attention to your HR can really prevent that if you use it correctly.  

A heartrate monitor can be a great tool for training. It can help you stay on top of when you are aerobic vs. anaerobic, which can help you stay on top of fueling in a big way. This can be a major tool in a long race, as it can help you see when you're burning the wrong type of fuel and won't be able to sustain your effort with the on-course nutrition (or what nutrition you're carrying).

The HR monitor can also help you with interval training, which can be used to build speed and endurance power.

However, for a HR monitor to be most effective, the person needs to know their specific zones. You can be of the exact same size, weight, and build as someone else and be completely different on your zones. Each person is different. I would recommend getting a VO2max test done so you can identify your zones and then train properly using them. There are some other ways to try to figure out VO2max without taking a treadmill test, but I prefer the actual treadmill test for accuracy. I also prefer it because it motivates a person to really commit to going "all in" on their test, which in turn I notice usually motivates the person to take the same approach to training afterwards.

My .02

I could offer up a number of physiological adaptive scientific based reason for HRM training.  But the fact remains when you have simple number that scores your workout, you try harder.  Once you start using one, you will get hooked on trying to raise the number.  Maybe that number is ave heart rate, percent of time in zone n, TSS, or the highly subjective calories burned.  It is a simple score of the how well you did it.  It provides positive feedback on how hard you worked. or how smart, or how long.   We are mostly type A persons looking to compete.  Even if we only compete with our selves.

HR zones are different for running and biking.  You can do the field LT tests as described in this BT article and then use the BT LT HR zone calculator.

A basic heart-rate monitor properly used is most likely the useful tool available for endurance athletes.

A heart-rate monitor will teach your body to burn fat for fuel as opposed to glycogen when you train in your fat-burning(aerobic)zone.

For example, the average marathoner has about enough glycogen stores to make it to about mile 20 or so in the marathon if they are running with an elevated heart-rate or in their(anaerobic)zone.

At that point they hit the invisible "wall" and their marathon becomes a death march.

However our bodies have enough fat stores to run three marathons or more.

The problem is that very few people realize this and so keep hitting the wall over and over again in marathons and half-ironman and ironman distance triathlons.

So is there more to a heart-rate monitor than just a way of telling you what your heart-rate is? You bet there is.

Find yourself a book by Doctor Phil Mafetone on heart-rate monitor training and you will learn a lot and become a more successful endurance athlete.