One of the features that comes standard on the Apple Watch that doesn’t come on the iPhone is Moon phase tracking. This lets you keep track of the Moon, whether it’s getting a full or new Moon, or anything in between.
If you've always wished your iPhone could do this too, you can use a new jailbreak tweak called MoonPhase by rainwolf. This will add a new icon to the right of the time in the Status Bar that reflects upon the current Moon phase.