First off, you don't have to. A heart rate monitor is not an essential training aid however in my opinion there are many advantages for all cyclist at all levels and if top cyclists use them for training and racing, that's good enough for me. Also with them starting at prices of less than £40 you won't get a better training aid or partner for the price AND it's a reliable partner.
What are the advantages of using an HRM?
First an HRM can help you be specific in your training. This is of particular advantage if you're short of time. Therefore, whether you want an aerobic session, an AT session, a recovery session, or you are performing intervals; your HRM will help you put in the correct effort.
An HRM can indicate if you need a rest, can indicate the early onset of illness, or can indicate over training.
An HRM will never let you down (unless the battery goes flat), unlike training partners that have family commitments, get injured, ill etc.
A HRM is a great addition to your turbo training, giving you something else to check and helping the time pass more quickly. One is also essential if you are going to perform pyramids etc. See turbo training.
Finally, there's something reassuring about a frequent little beep from your HRM when you're out on a ride by yourself. It may sound sad but I find a heart rate monitor the best training partner I have and I rarely go out training without it.
For a wide selection of the best heart rate monitors from POLAR, click the name.
How a HRM works
Working out your training levels