You raise a great question where does the CO2 go?
Plants take in CO2 > the O2 is released as Oxgen and the C or Carbon is left over as Biomass in the form of plant growth. Carbon is an essential element for life on earth and shouldn't be confused with CO2 or Carbon Dioxide which is a gas.
The earth is a closed loop system and it's constantly re-purposing the carbon along with other minerals providing the materials necessary for life on earth.
Fossil fuels are made of mainly hydrocarbons a product of carbon formed deep inside the earth. When they are burned they release the Carbon in the form of Carbon Dioxide.
Carbon Dioxide has long been part of our system and can be a product of things like respiration. CO2 is also needed in our ecosystem. The problem begins when we start to burn fossil fuels and at the same time cut down rain forests. Now we throw the system out of balance adding more CO2 into the air then our system can handle. It starts to build up in the atmosphere.
Plants may play a key role because of their ability to convert CO2 into C. Once the Carbons are out of the air they can be used again by the earth, thus restoring balance to the cycle.
I don't believe most people will have a practical use for this carbon. But it can be purposed for things like Fertilizer, Bio Fuel, and Food for animals. Some types of fish for example will eat algae. We may then eat the fish and everyone is happy!
Q: What do we do with the excess carbon?
A: We find beneficial ways to use it.