Sure, knowing how many watts you can put out is cool, but what’s the real reason you need a power meter? For starters, power is the only absolute measurement of your performance. It measures, in watts, how hard you’re pedaling, which lets you monitor your effort in real-time while riding. You'll also be able to check out all of your metrics, including how many calories you’ve burned once your ride is done.
If you’re only monitoring distance or speed, the intensity of the workout can greatly vary. Those metrics are heavily influenced by both terrain and riding conditions. So think about wearing a watch that only counts steps—it’ll measure a thousand steps whether you’re going upstairs or walking on flat ground, and this isn't a good indicator of intensity.
Heart rate is another metric to look at, but again, this information is different than power. It varies based on a number of factors, including hydration, sleep, stress, and caffeine consumption, so it's more of an indicator of stress level than riding intensity.
So, whether you’re a die-hard racer who’s looking to monitor their output both on, and off the bike, or if you’re looking to achieve a fitness goal and want to monitor the exact number of calories you’re burning on each ride, a power meter is the best way to do so.