If weight loss is your goal, determining this number will help you to burn as much fat as you possibly can, without deviation. So, with a heart rate monitor, you can determine your heart rate at that point. What that means is that you'll know, by looking down at the watch on your wrist, if you're burning fat. You'll be surprised at how easy that will feel and how long you can stay in the fat burning zone. Some monitors will even beep at you if your heart rate goes above or below that perfect fat-burning range.
Monitors are great motivators as well as omniscient personal trainers.
I wouldn't work out with out one because it's almost impossible to accurately guess if you're in the correct range for burning fat based on perceived exertion. With a monitor, you don't need to take these things into account. When my husband and I started working out with a monitor, I still hadn't really planned to change how I exercised. I was using him as a test subject, planning on using this experimental information for my clients. I figured if I go slower, I'll be regressing and decreasing my fitness level.