Mass inside the house complements the way fireplaces heat - radiantly. The masonry fireplace and chimney wall gets warm. The fireplace also heats up the other surfaces in the room - the walls, ceiling and floor - bringing up the "mean radiant temperature" so that people are more comfortable at cooler air temperatures and with good ventilation.

Mean radiant temperature - the average temperature of all the surfaces in a room - is roughly inversely proportional to air temperature. If the walls, windows, etc. average 60 degrees F and the air temperature has to be 72 degrees F to feel comfortable, you can reverse that and feel comfortable at 60 degrees F if you can bring up the mean radiant temperature to 72 degrees F.