Aside from keeping the warm air from escaping, the doors act to keep the hearth clean just as a fireplace screen used to do in the olden days. With the doors closed, ashes and embers don't scatter about the room, keeping the carpet or rug clean and scorch-free. The doors also have built in devices that divert air towards the bottom of the fireplace. These vents make the fire burn brighter and easier to light.
When thinking of modern versions of a fireplace necessity, the log holder comes to mind. The older styles did the job of holding wood beside the hearth but not too close to the fireplace to allow for ease in keeping a fire going with more wood. Now, not only will the holder do just that, it will come in two pieces to make the job even easier to bring the wood inside.