Your Apple Watch is a fantastic tool that can help you perform daily tasks on your iPhone. Notifications get filtered through it, you can respond to messages on it, and you can even use Apple Pay to purchase goods and services. Your Apple Watch is also a valuable asset when it comes to finding your lost iPhone, too.
Of course, you can use Find My iPhone via iCloud on the web or another iOS device to track your iPhone's location or to make your iPhone ring out loud so you can find it under a couch cushion or under the bed. But your Apple Watch should always be your first option since it's so easy to ping your lost iPhone right from your wrist.
You can't get an exact map location of your iPhone on the Apple Watch, but you can make it send out a beeping "I'm over here" alert. You have to either be within Bluetooth range or connected to the same Wi-Fi network (cellular data won't work if you have that model), so it's only good for finding your iPhone when it's misplaced around the house, not missing in town somewhere.
If your iPhone's speakers aren't working correctly or it's very dark out, you can even make the device's LED flash for a visual clue to where it is.
To ping your iPhone, swipe up from the bottom of the watch face to open the Control Center, then tap the "Ping iPhone" button (it looks like an iPhone ringing). Your iPhone will emit loud tones to help you locate it (even if it's on mute). You'll need to unlock your iPhone to stop the beeping.
If your iPhone's speakers aren't working at all or are muffled because of water or whatever, or if you're currently in a dark area, you can also make the LED flash on your iPhone with the Apple Watch. Just do the same as when pinging, except instead of tapping the icon, touch and hold it for about three seconds. Your iPhone's LED on the rear will start flashing until you unlock your iPhone.
If your iPhone is not in range of your Apple Watch, neither pinging or flashing will work, so you'll need to jump on iCloud or use the Find My iPhone app on another iOS device to track or ping it.