Ok I have 2 instructors telling me 2 diff things.
1 is telling me to use one so I can burn fat and stay in the "fat burning zone" while working out and even gave me a chart and I think I'm supposed to stay somewhere no higher than 146 according to my age. And if I'm too high I am no longer burning fat BUT it's good for my heart at the higher range.
2 is telling me no I don't need one. Don't hold back during exercise. What I really need is a calorie burner counter. Somethng that will tell me how many calories I burned.