Why do I need a separate heart rate monitor, when the band contains a wrist heart rate monitor?

The wrist heart rate monitor is convenient and accurate for instantaneous readings, or reading heart rate while at rest. But when you engage in more rigorous activity, the reading becomes less accurate with outside factors like your wrist movement, sweat, etc. The UA Heart Rate™ chest-worn monitor is a more robust heart rate technology designed to directly measure your heart's electrical impulses-providing continuous, highly accurate readings during physical activity.