It's very useful for aerobic exercise (running, swimming, cycling etc), where you want to pace yourself and keep your heart rate and/or speed/tempo in a certain range.
Now that you mention it, I have used it for this purpose myself! I turned it on when I was on the elliptical because I wanted to calibrate on what running in "Zone 2/3/4/5" felt like. And once I'd done that once I didn't really need to do it again, though I expect it will be helpful to recalibrate every few months as my capacity changes.
