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.
You should make sure that you have a power source close by your fireplace in order to plug in any fireplace blowers you decide to run. Usually, you can expect a blower to require one hundred and ten volts of power. You may need to have an electrical outlet installed so you do not end up with an extension cord running across the floor.