In very simple terms Carbon Dioxide is to plants, what Oxygen is to humans. They need it to survive. Just as poor air conditions can make humans feel unwell, lack of sufficient carbon dioxide can leave your marijuana plants struggling.
CO2 + Light Equals Food for Plants
Plants combine CO2 with light to produce the energy they need for growth and health. This process is known as photosynthesis. There is already enough CO2 in the air for plants to grow healthily so all growers absolutely need to add is light (and water). Increasing the level of CO2 can, however, result in bigger yields – but only when it's done properly.
Make sure everything else in your growing space is perfect before you think about adding CO2
In terms of value for money, you will achieve much more by eliminating common issues with growing spaces, than you ever can just be increasing CO2 levels. This includes ensuring optimum levels of light, water and nutrients, preventing mould and pest infestations and training your plants effectively. Only if you are running a perfect growing area should you even start to think about increasing CO2 levels.
Increasing CO2 Effectively Presents a Lot of Technical Challenges
First of all you need to decide on what method you are going to use to increase CO2. In reality
cheaper CO2 products are only likely to have any meaningful effect in the tiniest growing spaces (if anywhere), which means you need to prepared to make an up-front investment in either compressed CO2 or a CO2 generator. You will also need to increase light levels (otherwise the extra CO2 will just be wasted), which may mean upgrading your lights and reflectors. You will also need to ensure your grow area is completely sealed otherwise your expensive CO2 will just disappear into the atmosphere and that you can manage to keep a constant temperature of 30C to 35C and humidity of below 60%, at least when the plants are flowering (70% is OK during the vegetative period). Given that CO2 increases humidity, this is likely to involve running a dehumidifier.
In other words, you probably have to be a very dedicated and experienced grower to reap the benefits of using CO2 to increase yield. But if you think you have your setup dialled in then adding CO2 in whatever form should mean you see a greater yield from any plant.