The concept behind the vent-free fireplace is quite straightforward. When you burn gas instead of wood, you create a "smokeless†heat - - one that does not require ventilation, plus all the heat stays inside the home. A consequence of burning gas in the home however includes, among others, carbon dioxide, carbon monoxide, and sulfur dioxide.
Conventional fireplaces provide atmosphere for the home, but add very little in the way of heat, plus they are costly and take up lots of space.