Apple Watch uses the Wi-Fi and GPS from your iPhone, in addition to its own sensors for accelerometer, gyroscope, heart rate, barometer, tracking your physical activity and movement. There has been a lot of confusion, as to whether without the iPhone next to it, the Apple Watch could work independently like a fitness tracker.
As MacObserver has noted, model and activist Christy Turlington-Burns, who blogs about the Apple Watch on Apple's website, said in her most recent post, that the device can learn about the user's stride, if you've been using it long enough, to accurately track workouts without an iPhone.
After running the Apple Watch and your iPhone a couple of times, the 'Training' app knows more about your step, the activist wrote Christy Turlington Burns. So you can run on a treadmill or outside without your phone and still get a really accurate workout summary.
As someone who goes to the gym regularly, I would find it very uncomfortable running on the treadmill, doing push-ups and bench with a huge slab in my pocket.
As an iPhone 5S owner, I am constantly concerned about the device, that this accidentally slipping out of your tracksuit pocket and having to constantly interact with fitness apps on it.
A watch with a heart rate sensor, which continuously track pulse, while keeping everything about my runs and workouts stored while my iPhone is in my locker, it closes the discomfort and doubts I had about the Apple Watch.