I wear one because it can be an early warning for heat exhaustion. (As long as I haven't pushed so hard that I throw up I feel I haven't maxed out.) But if my heart rate goes up on a climb and doesn't come back down on the way downhill then I know I need to take it easy till it does. I short if it goes up on a climb and then returns to 120 to 130 in a few minutes I know I am fine. If it gets to 165 and ten minutes later it is still 155 in the flats I need to gear down and cool down. But that is just me. Some of my friends never get to 160 even on a climb.