Why do farmers plow under their crop?

The only time I've ever seen a farmer plow under a viable crop was when the market was so bad that it would cost more to harvest than it would sell for. The plowed under crop will break down and provide organic fertilizer, so at least it will have some value then.

Other times the farmers plant green crops with the sole intention of plowing them under to improve the land. Legumes and rye grass are popular for that use.