They are a valuable tool in training for me. Once you know your max. sustainable HR, you can know what HR to maintain in order to finish strong. Cadence is another tool that for me I continually check on as its easy for me to be a little slower than I need to be and when I check myself and increase the cadence I feel better and usually pick up a little speed too. One other benefit is being able to post my rides on line and compare routes and performances with what I have eaten before and during the ride. There are others too.
Good luck