iPhone: By itself, the flash bulb on your iPhone isn't particularly powerful. Set off a lot of them at once though, and you can light up an entire room.
External Flash is a clever little iOS utility that can chain together up to sixteen iOS devices on the same Wi-Fi network, and trigger all of their flash bulbs simultaneously. This could be useful when taking group photos in dark rooms, or at an occasion like a graduation where several friends are taking photos of the same event. Another use case though would be to pair a few iPhones up with a DIY light box to take well-lit product shots for Ebay or Gumtree ads.
Unfortunately, the app doesn't currently support the front-facing camera. It would be great to use a second phone's flash to light a selfie; perhaps this will come in a future update.
This isn't an app you'll keep on your home screen and use every day, but for specific situations, it's a useful option.