Every program worth its salt will have the athlete train with a specific heart rate depending on the purpose of the workout (VO2Max/lactate threshold). This number depends on your max heart rate (MHR) and knowing what your MHR is will help you optimize your training as a result.
VO2Max determines your fitness shape (cardio). It is also an indication of how well your heart’s ability to pump blood is, as well as your recovery rate. This is also related to your lactate threshold.