When you are wearing your AirPods, you can know who is calling without looking at your phone. You just need to turn on a setting that will have Siri read out the caller to you over your headphones. You can also turn this on for any car Bluetooth system at the same time.
Go into Settings -> Phone -> Announce Calls -> Tap on either Headphones Only or Headphones & Car.
When you get a call the headphones will read out the contact name to you over the AirPods. If it is a caller that is not in your contacts it should read out the phone number.
This will prevent you from answering a call in a goofy voice when it is actually your boss or it can help you avoid picking up a call you don't want.
When you are busy, on a run or just can't reach your iPhone, this is a handy way to know who is calling over your AirPods.