Knowing your heart rate and being able to relate it to perceived exertion level and other factors like rest and diet is a valuable tool if used in concert with the other factors. It can be nearly useless and maybe even hazardous if used solely. Heart rate is merely data until it is merged with the other facts to become information.
I use a heart rate monitor because it is more convenient than manually taking my pulse. One could say it is also a safer method of getting the heart rate than taking a hand off the handle bars to get the data.