Because they are unbiased, and they better help you to understand the power you are putting through the bike. Years ago at the Science Museum they had an experiment to explain Watts directly. They had hooked up a static bike to a three 30watt light bulbs followed by a 240watt electric fire on a circuit. My daughter, aged 5, was really impressed as other people who tried the experiment managed to get the light bulbs to come on, but not the electric fire.
So the maths here was: 30watt + 30watt + 30watt + 240watt = 330watt
So I managed to produce 330watts, for a very short period of time, as the electric fire was set directly above the rider – we then had to ‘toddle’ off to the cafe for a cool drink! The lesson learned here is: Watts are real-time unbiased statistics, unlike heart rate where statistics differ due to physiological cost. And show-off to your children as much a possible when they’re young, as they’ll think you’re cool; as when they’re older they won’t!
Now consider that Professional cyclists can sustain an average 300-350 watts for a 4 hour stage of the Tour de France, and an average person might sustain much less than this for a spin class. That’s really cool.
Watts the point?
Power (Watts) = Force x Time
Watts is the energy required to move a mass, in a time period.
The point is that the statistics generated by Watts from your Power Meter, can now be used to work out your FTP to enable you to use the Coggan training zones. In turn, put simply, the training zones directly determine whether you are training to become a sprinter or an endurance cyclist. Working predominantly in Zone 5 each time you train will cause ‘adaptation’, which in this case will be bulking up your thighs, to turn your body into that of a sprinter. Whereas Zone 2 training will cause an ‘adaption’ to a lean physical appearance as the body adapts for endurance.