The aim of a cover crop is to minimise exposed soils, whicht can lead to soil erosion and loss of soil organic matter. Additionally, a cover crop can catch nitrogen from the soil. In this case the name 'catch crop' is used, since it avoids the leaching of nitrogen.
Cover crops can be sawn on different moments. Sawing after the main crop is most common. In combination with a late harvest this gives a limited choice in crops and the later they are sawn, the porer the development of the cover crop. That is what makes sawing the cover crop between the main crop interesting (e.g. grass between mais). In this case the cover crop can be sawn together or after the main crop. This gives an early growth of the cover crop and a better development of this crop. In addition the cover crop protects the soil during the harvest of the main crop.