Well, we do not need to add CO2 gas.
Plants will still grow.
But they grow slower and they compete with each other stronger for CO2 if it's a limiting resource.
In non CO2 tanks, some wimpy plants that are poor competitors, or have higher CO2 demands or need more light before they can start taking up the same amount of CO2.................do not fair well.
To remove this competition.......we add CO2 gas.
Now we can grow pretty much any combo of plants species in the aquarium.
The other big reason is to increase the rate/speed of growth. About 10-20X faster.
"Amphibious" nature over a season is also mentioned and quite correct.
In natural systems, there are plenty that are loaded with CO2, mostly karst limestone fed springs.
I've found a few volcanic springs with rich CO2 as well.
Hat Creek in CA, USA.
A CO2 rich stream with few plants, Santa Ynez River:
Bonita Srpings, Mato Grosso Brasil(do a google search)
Pupu Springs, New Zealand.
More volcanic CO2 rich streams in Lassen's national Park, CA, USA
Florida has hundreds of CO2 rich spring feed systems: