I think it may help you realize you aren't getting as hard of a workout from riding and maybe adjust your eating? Or maybe you are getting a hard workout and not fueling enough.

I found I gained a lot of weight when I didn't run because I thought I was riding hard and therefore I was eating a lot. My perception of how much 4 hours of riding really was working was off. I don't know if the HRM helps, I have lost 25 lbs since getting mine. I also started running again and have paid more attention to my food choices. It can be a tool to add to your arsenal but won't get you there alone.