Numerous studies (1,2) have shown that breathing rate (BR) is much more closely correlated with your true internal effort than heart rate. This is because both BR and the exertion you perceive (called Rate of Perceived Exertion in sports science) are determined by the same central command signals sent out by the brain to the muscles when you engage in hard exercise. Heart rate is more influenced by various external factors like caffeine, lack of sleep, temperature, humidity and stress, which are some of the problems with heart rate-based training. The Frontier X2 can also be configured to alert you when you cross certain breathing rate thresholds, so you're alerted with a buzz when crossing into a higher effort zone than what you had planned for the workout. This can be used to optimize your training in different effort zones.
During a race or event, the alert based on Breathing Rate will warn you if you are running or cycling at an unsustainable pace, and by slowing down and breathing deeper, you will be able to avoid hitting the wall in such situations.