By definition, a cover crop is a quick-growing, leafy plant that is grown to enrich the soil. The answer to this question greatly depends on what goal you wish to accomplish with your cover crop. Many varieties of cover crops are members of the legume family.
Legumes form a symbiotic relationship with soil dwelling bacteria that can fixate nitrogen.
Besides nitrogen, buckwheat is a cover crop that actually accumulates phosphorous and yellow blossom sweet clover can use its taproot to bring buried minerals to the soil surface. By planting a cover crop with a strong root system and a dense growth habit, you can choke out weeds. Barren, unoccupied soil is very prone to erosion. Erosion from wind or rain causes the top layer of soil to wash away. The top layer of soil houses many nutrients, along with beneficial bacteria and fungi that aid in plant growth.