It's mainly for people that for some reason can't add additional air conditioning, but have the power to bring in 1+ tonnage of water chilling. Or they have access to well water.
I know of people that do huge grow ops (totally non-med, stealing power, etc etc) and they intake/exhaust outside air. They work with familiar strains and can pull out crazy yields, but they do have to battle insects and PM constantly.
One of the main benefits of a CGE is the prevention of a pest problem. I don't think anyone can really quantitatively tell you the increased yield and quality by running enriched CO2 environment, but we all know it works and works well.
Lastly, depending on your latitude, CGE allows you to run year around. Most open air growers have to cycle down lights or stop completely during the summer months due to heat.
If you're in higher altitudes, swamp coolers work pretty well.