I've been using an HRM for about 1.5 years now and really have come to rely on it. I'm not a number cruncher so I'm sure someone else here will give a more scientific answer but I've found it to be helpful for the following:
1.) Accurate calorie burn tracking - If you're riding to get in shape and use this in conjunction with a calorie counter app you can really dial in the perfect work-out regemine to lose weight, gain weight, or whatever your ultimate fitness goal is. This also helps remind you to consume more calories on a long ride.
2.) Understanding your target HR zones and tailoring your riding to your training goals. I know where my zones are and have actually seen my LT improve drastically over the last year so I know I can put in more effort for a longer period of time. Using my HRM helps me time LT intervals and keeps me in check when I'm supposed to be on a recovery ride. Set it so the alarm goes off when you get above your target zone and you'll get instant feedback (in addition to your body telling you) that you're working hard. Then you can use the data after the ride to analyze how much time you spent in each zone. Use this to set training goals and milestones. IE.) You know you're getting stronger when you can maintain a higher BPM for a longer period of time.
I know there's a lot more to it, but I really like having that instantaneous feedback and the ability to set benchmarks and goals based on the info I get from the HRM.
