I believe what it does is economically incentivize more people to build renewable energy plants by paying them more for each kWh they generate. If you pay to cover one ton of CO2 you are raising the price paid for renewable energy, such that more renewable plants will be built, such that some future energy need will pull from those plants instead, ultimately generating the total amount of energy the world demands but with one ton less CO2 released.

I'm far from an expert though, hopefully someone will correct me if I'm wrong.