The control system controls the turbine during operation, for example how much it should pitch and yaw.
By controlling the turbine, we can reduce the load on it. Reduced loads make it possible to construct a lighter turbine with longer blades. By doing so, we can reduce the foundation needed for the wind turbine and - since this is a major cost during construction of a wind farm - reduce the overall cost of the wind farm. It also makes it easier to transport the turbines to the site.
Smart controls also reduce the wear and tear of the turbine during operation.