The power of this question is that it forces the person answering it to think outside the box. It invites them to bring up whatever important topic we’ve forgotten. It demonstrates the value of their input and ensures their engaged contribution. It holds up a mirror to their thought process and provides a moment for self reflection.
Often the answers people give end up being the most important take away from the conversation. Next time you’re talking about something important, give it a try and let me know how it goes.