I own an HRM and a FitBit Zip. The FitBit monitors my general level of activity. One day I was working at an office that had a big kitchen with cookies downstairs and while I felt like I wasn't very active, I kept going downstairs for snacks and water and I actually got in 10k steps thanks to my sweet tooth and glasses of water. The step count is pretty accurate, but I don't trust the calorie information it gives me. It just keeps me on track in general.
I use my HRM when I'm running or doing another high heart rate activity like Zumba. Everyone puts in a different amount of effort into the same exercise, so and HRM will give you an exact number and show changes over time. You can also use your HRM to exercise within aerobic/anaerobic zones based on your basal heart rates, if that's your thing, but I just like having data that proves how hard I'm working out. I also just find that information interesting -- even when I'm running at the same speed on a treadmill, I can go back and see when my heart rate spiked and have proof that I'm pushing myself. I use a bluetooth HRM that can talk to my phone and I sync that with Endomondo to get graphs. (I wish I could sync it here...) I like that better than having a watch that records the data, since I have a Mac and could never get the data off my Polar HRM.