When you give your plants all the water, nutients, proper ph and lots of light the limiting factor becomes C02. Your plants use C02 to compleate photosynthesis.
So adding C02 is like adding anouther nutient. When I first used C02 I almost tripled my yield although part of the gain was better care of my plants as I was a nubie then and some of the gain could be atributed to better feeding and watering. It's less important to very young plants but it is very efective. Be sure you have lots of air circulation as the more air that passes over the leaves the faster they can resperate.