With iPhone Battery On WATCH you have the charge level of your iPhone always in sight on your Apple watch.
If you swipe up on your Apple Watch watch face you can have a direct glance at the charge level of your iPhone.

Very handy when you are out, if your iPhone is in your bag or if you just plugged it on the charging cable and you want to keep track of its charging process. Its also a neat feature if you want check you iPhones charge level discreet, while you are in a meeting.