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.
One more indispensable attribute of a fireplace is a mantels for fireplaces. Very often it is made of wood, typically of oak or walnut, and then it gets smoothed and polished or darkened by time and soot. Of course, the cost of fireplaces and fireplace accessories is quite high, mostly because the price of finishes and high quality materials. A fireplace is a bright, colorful interior decoration, a symbol of stability and warmth. And to some extent it is a luxury item, and as you know the real luxury just can not be cheap.