Plants need three things to live,
Light
CO2
And ......Nutrients.
Water is not an issue here, the plants live in water.
If you don't have enough of one of these, then it will not matter how much of the others you add, it will not produce more growth.
So adding more CO2 will allow the plant to use the light and the nutrients much better. The amount of CO2 is relatively low in your tank to suit the plant's needs(they will grow without it, but adding really helps a great deal). Using CO2 gas helps this problem.
Plants are roughly 40% dry weight carbon. So adding it make this primary element much easier to supply.
Regards,
Tom Barr