HRM are very useful, IF you know how to use them and you are actually training for running.
First thing to realize is that the old 220-age thing is a rule of thumb, not written in stone. My HB has always been higher the "normal". After going for a test at a University, I learned that I have higher then normal HB. Normal for me.
Second, most people use it as a crutch. It's a tool, learn to use it as such. Mine will tell my distance, elevation, HR, pace, speed, etc. Looking at when I was training for a sub 15 minutes 5K, it helped a lot for pace and making sure i wasn't overdoing it. But useless on hill day.
Also, on a personal level, a question like that is equivalent to "I have a weight belt, how do I use it". I comes with instructions, read that. Learning to program that damn thing, is another question altogether.