A power meter measures how hard you’re working regardless of the terrain, the weather, your fitness, or any other factor, and it will give you figures that you can compare over time to accurately gauge your progress. Your speed over a set course can be affected by a headwind or tailwind, for an example, giving you a false impression of your fitness, but measuring power it tells you exactly what your output is.
With a power meter it takes the guesswork out of your training efforts, allowing your training time to be efficient. You can plan exactly what you need to work on, then go out and do it, Get your cues from the numbers on your bike computer or indoors with your PC or Mac.
You can use a power meter to help pace
A power meter helps you identify your weaknesses. If your sprint power are lagging behind other areas, there’s no hiding from the numbers so you know exactly what you need to work on in your power profile.
A power meter can if you when you need to rest. For example, if your heart rate is higher than normal for a specific power output, it could be that you need a recovery day.