I need some objective means of quantifying my effort (power) vs heart rate and recovery times. It's important to me simply because 10 years ago I suffered an MI (97% obstruction of the left anterior descending) that resulted in a sudden cardiac arrest. At the time, I was in superb shape and felt fine. Right up until the lights went out. Fortunately for me, after a double by-pass, I recovered with minimal residual damage to my myocardium, and was back on the bike in less than 6 weeks post-op with the approval (and actual encouragement) of my surgeon and cardiologist.
So... At least one ride a month is a sort of cardiac stress test to keep track and that requires a power meter. The rest of the time, I just like playing with the numbers.