Cover crops have many beneficial uses, but different species and mixtures will have particular strengths.  For example, if the objective is adding organic matter or providing weed suppression, then cereal rye or similar choices may be best.  If providing N for a following crop is the main goal, then an all-legume cover crop such as clover or vetch is probably the correct choice.  More information on the potential beneficial uses of cover crops and targeting for specific soil health and agronomic benefits can be found here:

Beneficial Uses of Cover Crops