Simply put, power meters provide an objective measure of your riding effort... unlike heart rate (which is susceptible to a slew of variables), power meters provide reliable feedback regardless of terrain, weather conditions, etc. Once you get a feel for your own personal "numbers," this information is critical to successfully pacing yourself in training and racing.
In addition to their success at the highest levels, Stages is also widely regarded to be the best value in the power meter market today. Riding with power is far more affordable than it used to be... check out Stages to see their full product line including closeout deals.
Much more to come, but for now check out the video below to learn more about Stages and Team Sky.