Heart rate monitors are your friend when exercising, as they are useful for many reasons. First of all, they are useful tools for people that are older or who have certain conditions. They are able to monitor their heart rate and not get themselves into dangerous territory while exercising.
They are also simply convenient tools for exercising in general, and you can count on them to be very accurate. In other words, you're not getting some ballpark figure. You're getting accurate results while working out, enabling you to have a more efficient aerobic workout.
You know that intensity is key to a better workout, and the heart rate monitor helps you better gauge intensity. Moreover, it helps you recognize whether or not your fitness level is getting better as time goes by. You can always be on the lookout to make sure you're hitting your target heart rate. More fitness tips can be seen here: http://www.menshealth.com/fitness/tips-to-transform-your-body
I've never had one of these heart rate monitors, but I have used one at a gym that was part of the equipment. I'm not sure if this specific monitor was as accurate as one that you might use that wasn't connected to a machine at the gym. I think I would rather purchase one anyway so that I could use it wherever I'm working out.