Do I need my phone when I use my Apple Watch?
Processing of your day happens on your Watch when you start your day there.
While you can technically leave your phone behind, I don't recommend doing so for two reasons:
- The Watch battery is dramatically improved with your phone still in your pocket. Watch is smart and will use your phone's GPS instead of its own when it can, dramatically saving battery. With your phone in a pocket you can expect the Watch to easily last all day, where-as without the phone you'll likely see closer to 6 or 7hrs of battery before it dies.
- The GPS in the iPhone is also a bit more accurate, especially with altitude data. Keeping your phone with you means you get the most accurate data. In the thorough testing I did of Slopes on Watch I found keeping the iPhone near by ensured accurate metrics, and also had an easier time keeping reliable data.