Single headphone usage is a feature. The AirPods can detect that only one is present in the ear and switch from stereo to mono output. Here are a couple use cases:

* To listen to the environment as well.

* To take phone calls.

* To extend the battery life of continuous listening by listening to one, charging the other, and then switch.

To listen for hazards while outside. I don't know about inside.

I do it when I chain calls and I don't have batteries anymore. I can charge one while using the other, then swap. The battery is great if you're only listening, but as soon as you start using the microphone it goes down pretty fast (a bit more than one hour maybe?).

In addition to the other reasons mentioned, I sometimes wear one at work on the side facing the wall of my office so I look more approachable.

In my case, I lost one of them.

Podcasts, audiobooks, or taking phone calls.

halving the wear on the batteries?