Carbon Dioxide or CO2 is used in reef aquariums to dissolve media and release calicium into the saltwater aquarium. As almost half the dry matter of a plant is cabon dioxide it is a key component to making a successful freshwater planted aquarium.
In the marine or saltwater aquarium, CO2 is used to help to lower the pH of calcium reactors effluent in order to dissolve hydro carbonate media into bio-available calcium molecules. Calcium is then utilized by corals and other marine organisms mainly for skeleton building process.
In your freshwater aquarium environment, CO2 will enhance the ability to successfully keep live aquatic plants. This is because as green plants photosynthesize, they utilize carbon dioxide. Without CO2, live aquarium plants will have a hard time thriving in the closed aquarium system.