Here's how Apple describes the M7 coprocessor:
The new M7 coprocessor is like a sidekick to the A7 chip. It’s designed specifically to measure motion data from the accelerometer, gyroscope, and compass — a task that would normally fall to the A7 chip. But M7 is much more efficient at it. Now fitness apps that track physical activity can access that data from the M7 coprocessor without constantly engaging the A7 chip. So they require less battery power. M7 knows when you’re walking, running, or even driving. For example, Maps switches from driving to walking turn-by-turn navigation if, say, you park and continue on foot. Since M7 can tell when you’re in a moving vehicle, iPhone 5s won’t ask you to join Wi-Fi networks you pass by. And if your phone hasn’t moved for a while, like when you’re asleep, M7 reduces network pinging to spare your battery.Right now, only a small subset of the general population is engaged with activity self-tracking using wearable gadgets. Wearable activity trackers include Fitbit, Nike+ Fuelband, Jawbone UP, Misfit Shine, and many others. In order for these gadgets to be effective, you have to remember to wear them.
I've been an early adopter of these wearable fitness trackers. I've used a variety of them over the years and I have personally gravitated towards gadgets that I can always wear on my wrist. This way, I don't have to remember to put it on. Right now, I wear the Misfit Shine on my wrist. I sleep with it. I shower with it. It's always on my wrist. This way, I never forget to wear my fitness tracker. When I used to clip a digital pedometer or Fitbit on my pants, I would frequently forget to clip it on before leaving the house. I've also met many people who have misplaced or lost their waist-worn devices.
The Apple iPhone 5s is going to be a game-changing device for people who want to live healthier lives. If they're not actively using fitness trackers, they'll have the ability to use their iPhone to track and monitor their physical activity. This feedback may be enough to motivate them to make sustainable behavior changes that can lead to healthier lives.
Does this mean that you should not invest in a wearable activity tracker if you buy the new iPhone? No. The data from a dedicated wearable tracker is going to be more consistent since you can wear some of these to sleep, while taking a shower, while swimming, etc. If you're going to track your activity, why not track all of your activity? This is how you'll learn the most about your own body and your fitness level.