In their natural state, rocks naturally absorb atmospheric carbon dioxide as they break down in the environment. The greater the number of pieces they break down into, the greater the total surface area available for carbon sequestration, and thus the greater the amount of CO2 that can be absorbed. Most rocks take a long time to fall into small pieces on their own, however. Additionally, in order for all those pieces to absorb CO2, they have to be laid out across a large area where they're all exposed to the atmosphere. That's where enhanced rock weathering comes in.