CO2 is carbon. Carbon is the basic building block from which all life is created. The flourish provides mineral nutrients. CO2 is also a nutrient but of a different sort. Co2 gas gives plants a usable source of carbon. Mineral nutrients & carbon are converted by the plant into actual plant matter. Without a source of carbon, little or no growth can take place.
Low light tanks very rarely benefit from additional co2. The general way it works is the more light you have, the more energy the plant will have at it's disposal. The more energy a plant has to use the faster it can convert mineral nutrients & co2 into plant matter. A deficiency in any of these , mineral nutrients, carbon or light will result in slower plant growth & possibly plant health if it is a severe shortage of any one of those things.
Unless you have lots of plants & lots of light I probably wouldn't be too worried about adding co2. You could always try using Seachem Excel instead of Flourish. It's essentially Flourish but with a liquid source of carbon for the plants to utilise. I've heard of people having trouble with it through over dosing so if you try the Excel just make sure you don't overdose. It can put your fish at risk of oxygen starvation.