Growing the same crop in the same place for many years in a row disproportionately depletes the soil of certain nutrients. With rotation, a crop that leaches the soil of one kind of nutrient is followed during the next growing season by a dissimilar crop that returns that nutrient to the soil or draws a different ratio of nutrients.
Or you can get this Planning Manual that provide an in-depth review of the applications of crop rotation-including improving soil quality and health, and managing pests, diseases, and weeds.