When to use this analogy?
1) Someone is having trouble understanding how faith demands action.
2) When explaining how emotion comes from information. Emotion is a response to what happens in the mind.
A Fireman knocks on your door. When you answer, he tells you that your house is on fire! If you were to respond by thanking him, closing the door, then going back to watching TV, did you believe the fireman? No. Of course not. Believing the fireman requires understanding your house is on fire, putting your faith in the information and responding.
How would you respond? Perhaps a sudden terror would rise up in your gut and your mind would race with what you needed to do. Next you would get everyone out as quickly as possible. You believed the fireman, which caused you to act. Notice that without the information the fireman gave you there would be no emotional response.