Before revision: Unlike the underside of a heat-sink which needs to be smooth for maximum surface area contact with the heat-source that it cools, the fins benefit from this roughness because it increases the fins' surface area which serves to provide more area on which to release heat into the ambient environment.
After revision: Unlike the underside of a heat-sink which needs to be smooth for maximum surface area contact with the heat-source that it cools, the fins benefit from this roughness because it increases the fins' surface area on which to release heat into the ambient environment.