You don't "need" CO2. I used to have live plants (swords, vallisneria) in my kribensis tanks for years before I had even heard of CO2 injection and they did just fine (by which I mean they survived, and grew a bit, especially the vals, although I did have to add an iron supplement to keep the swords alive in the long run). Many stem plant species apparently grow like weeds without either good light or CO2. And some plants are specifically adapted to low CO2 concentrations in the water: floating plants won't need CO2 in the water as they take their CO2 from the air, and lobelias take their CO2 from the substrate (you need a fairly thick layer of substrate for this, though).
However, a DIY CO2 generator is extremely easy to make (a few minutes really) and costs almost nothing, so you might want to use CO2. The difference in plant growth is absolutely stunning, and people report huge improvements as a result of CO2 even in fairly dimly lit tanks (which makes sense, since plants no longer need to expend energy on efficient CO2 intake and/or bicarbonate utilization, and can focus more energy on efficient photosynthesis).