1. An off season focus on an aerobic activity (other than cycling) to keep limber, keep one's cardiovascular system tuned up, and avoid burnout from a single minded focus onbicycling.
2. Cross training to maximize performance across several disciplines - such as competing in a triathlon or biathlon.

Cross Discipline "Competitive Level" Training

Q. "I will have to travel for a week without my bike. I intend to run three or four times this week. Can I use the same value for cycling heart rate for the run and send it to the training peaks program I use to monitor my cycling trainings?"

Can you use the same numbers (HR in this case) to measure your aerobic output and compare your training levels? Numerous studies have been done in triathletes. This article answers the question in fairly technical (physiologic terminology and abbreviations). But it concludes that HR based training zones CANNOT be translated from cycling to running. "...From the results of this study, it is recommended that the triathlete perform sport-specific testing to assess training zones for cycling and running." It then goes on to suggest that measuring wattage as a training tool is preferable to HR. "... For this reason, monitoring wattage during steady-state exercise may be more appropriate than monitoring HR and RR."