Cover crops are a good thing. They let the soil rest while many inject nitrogen in the soil. They do add green manure to the soil when plowed under, or can be pulled and added to the compost pile, depending on what cover is used. They keep the soil stable, from washing or blowing away.
You do have to be careful what cover you use. Vetch can be somewhat invasive, so till it before it makes seeds. I think planting a usuable cover crop works well, sweet peas or greens for example as fall crops. Also many of the greens could be planted as a cover in the spring, and produce an edible product at the same time. Arugula is great IMO. I planted winter rye as a cover one year, and found that it was difficult to plow and break up the following year. So investigate and make a sound choice before plant some cover crop.