Think of biochar like humans helping plants keep the carbon out of the atmosphere. Plants are good at capturing carbon, but what happens when they die? Hell, what about all the leaves they shed? When something rots, it releases a mix of CO2 and methane (which decomposes into CO2). The idea of biochar is that it's a way of sequestering the carbon that plants captured. For an example, you make an algae pond, harvest the algae, dry it, char it, bury it. That's carbon that's not going back into the atmosphere anytime soon, whereas if it was left to rot, it'd eventually wind back up in the atmosphere. You're taking the carbon the plants captured, and processing it in a way that makes it easier to sequester.