[MRM]

For ordinary use interior doors should be open. Depending on the house, keeping the interior doors open is either helpful or essential. You really should keep them open for air exchange unless you need to keep young animals in or out.

But at night and when you're away, closing the doors really does help if there is a fire. It won't stop a fire that totally engulfs the house, but it really protects the interior of the room to have the door closed. That's why if there is a fire and your door is closed you're supposed to put your hand on it to see if its hot. There could be an inferno on one side of the door and the other side not even touched yet.