Cadence measurement (the speed at which you pedals go round) has become an increasingly popular training tool.
Why? Because optimising your cadence during rides can really help to improve your efficiency and endurance.
Although very few workouts are purely cadence based, it's pretty important in getting the right results from your workout – it’s all about simulating different scenarios you will encounter when riding your bike.
During your training the most important thing to hit is your target heart rate / power – your cadence target is simply for guidance in order to suggest the scenario you are simulating.
For example, with a target cadence of 80 and a heart rate of 120bpm you should be pedalling a very light gear with almost no effort.
To take another example, pedalling at 60rpm could mean you are either pedalling very slowly with no effort or you are pedalling very slowly because you are riding up an very steep hill. The two different scenarios would produce very different heart rates.