Cover crops stabilize and add nutrients.
Cover crops cover the soil and prevent excess nitrogen from leaching.
Plants take up excess soil nitrogen and return it to the soil when they are turned under.
In addition, legumes, such as clover, fix atmospheric nitrogen in special nodules on their roots, in collaboration with Rhizobium bacteria.
This nitrogen can be used by the next crop you grow. In this way, cover crops act as a bank account for nutrients, returning the investment with interest when you turn them under.