Sometimes I leave messages that include an address or a phone number for someone else. What happens if you don't call back immediately and I no longer have access to that address or phone number? Or I'm now unavailable because I just boarded a plane/entered a dead zone/am watching a movie at a theatre/my phone ran out of battery/I'm asleep and turned the ringer off/I'm driving in a state where it is illegal to talk on the phone while driving and I don't have Bluetooth/I was kidnapped by ISIS and they took my phone/etc.

Sometimes you NEED to leave a message. But you'll never know what it was.