Insects often hide in garden debris. If that debris is worked into the soil, insects will be less likely to survive the winter. Diseases are also less likely to overwinter if old plants are worked under. Also, the garden debris will increase the organic matter content of the soil. Working the debris into the soil is often easier if you mow the old vegetable plants several times to reduce the size of the debris.
Fall is an excellent time to add organic matter. Not only are organic materials usually more available in the fall (leaves, rotten hay or silage, grass clippings) but fresher materials can be added in the fall than in the spring because there is more time for materials to break down before planting. As a general rule, add 2 inches of organic material to the surface of the soil and till it in. Be careful not to overtill. You should end up with particles like grape nuts or larger. If you work your garden into the consistency of dust, you have destroyed the soil structure. (Ward Upham)