Cover crops are "green manure" that help manage soil fertility. Though there are many other benefits (and types) basically these members of the pea family return nitrogen to the soil and by containing erosion and controlling water usage prepare the soil for your next crop.
The greens from cover crops can be cut into the soil for extra nutrients and many are excellent animal feed treats. Every year I plant a hefty crop of bright green fava beans, the darlings of Mediterranean style spring salads, fresh crostini and pastas.
Now in my little raised bed with heavily amended soil I can't say I really need a cover crop. My beds could sit idle through the winter waiting for spring's frost date and my hearty summer tomato crop. But, I love fava beans.