Fireplaces can work without a glass door. But having a glass door brings in several advantages. For one thing, it provides a barrier between the hot inside of the fireplace and the room. This is a great safety feature to have, especially if there are kids or pets in the house. Also, studies show that closed fireplaces are more energy-efficient than open-hearth fireplaces. Making the fireplace more efficient will result in long term savings as you can considerably reduce your monthly utility cost. Another obvious benefit is that glass doors can prevent smoke and ash dust from getting inside the room.