not a must at all. it simply measures how fast your heart rate is - you can certainly ride a bike and train without one. there are benefits to using one, because you can train in certain zones without any guess work. you'll also know if you are over extending yourself. might be good if you have a heart condition.
there is a medical exam you can take to determine your max HR, but for most of us we use the 220-age = max HR. i'm 27 years old so my HR is 220-27 = 193. it's not perfect but it's in the ball park.
If you want to use a structured training programs, they are very helpful. If you just want to ride and have fun, there's not much point. On a long ride they are good at keeping you from exercising too hard.
late really sum it up. Nevertheless, since you're reading this column, and not touring or pleasure ride, I'd reccommend that you get one. To me, the two most important data I need are HR and cadence. I don't compete anymore, but I still wear the chest transmitter on most rides, even on touring trip. After the HR and the cadence, then it's speed and trip meter.
"there is a medical exam you can take to determine your max HR, but for most of us we use the 220-age = max HR. i'm 27 years old so my HR is 220-27 = 193. it's not perfect but it's in the ball park."
BTW It's not in the ball park, but a safe starting point for somone who hasn't taken max HR test under medical supervision. I'm not in shape competition wise, yet my max HR is over 10 beats above the 220-age. Imagine the max HR of Lance. Testing for max HR without medical supervision can be fatal.