MacRumors have an article on how to setup Live Listen with AirPods. Live Listen has been in iOS since 2014 and allows your iOS device to be used as microphone for compatible hearing aids but with iOS 12 the feature works with AirPods as well.

Whilst this might not seem that useful if you’re not hard of hearing, but as long as your AirPods are connected to your iPhone you can listening in to whatever is going on where your iPhone is.