The most common reason for seeding a cover crop is to establish cover onto a tilled area following harvest of the previous crop, or onto a disturbed area for a "critical seeding", as soon as possible after tillage or disturbance. The reason for establishing the cover crop is to stabilize the exposed surface of bare soil to prevent erosion.
Wells, Kenneth L. and Bitzer, Morris J., "Seeding Cover Crops in Kentucky" (1996). Soil Science News and Views. 128.