Cover crops have mulitple benefits.  They can prevent soil erosion from rain and wind while improving crop yields.  In recent Kentucky experiments, winter cover used with no-till planting markedly increased corn yield. For instance, 3-year average continuous corn yields were 8 bushels per acre greater when planted into a winter cover of rye and 25 bushels per acre greater when planted into hairy vetch than yields from plots without winter cover. The significantly higher yield in the hairy vetch cover plots was due primarily to the extra nitrogen (N) that this legume provides. All plots in the experiments had received 88 pounds of fertilizer N. (http://www.agry.purdue.edu/ext/forages/publications/ay247.htm).