One reason is that plants in one family often are attacked by the same pests and diseases. Rotating out of a particular space, and planting crops from a different family there instead, can help reduce the buildup of diseases and pests that attack crops in one plant family.
Another reason is that plants in one family often make similar nutrient demands on the soil. Jessica Strickland of North Carolina Cooperative Extension, in a May 2013 article, wrote:
"Vegetables in the same family are similar in the amount of nutrients they extract from the soil, so over time planting the same vegetables in the same spot can reduce certain nutrients in the soil. If the same family of vegetables is planted every year in the same location, insect and disease problems continue to increase and soil fertility drops. Using pesticides and fertilizer could provide little help but over time they would not be able to keep up with the increasing problems."Also, rotating to some particular crops can help reduce a pest problem that already has built up to damaging levels. An example pest is root knot nematodes, which can lower productivity of a crop pretty dramatically - if they don't actually kill the plants outright. A population of these soil-dwelling pests can be lowered by planting a bed solidly in one of the nematode-repelling marigolds or in a grass-family crop like rye, wheat, or oats.
What I didn't know until recently is the effect of crop rotation on the diversity of soil microbial life, the maintenance of which is so integral to successful organic gardens. In the Science Daily article "Why crop rotation works: Change in crop species causes shift in soil microbes", Professor Philip Poole of the John Innes Centre in England is quoted as saying,
"Changing the crop species massively changes the content of microbes in the soil, which in turn helps the plant to acquire nutrients, regulate growth and protect itself against pests and diseases, boosting yield."Professor Poole added: "While continued planting of one species in monoculture pulls the soil in one direction, rotating to a different one benefits soil health."
Yet another good reason to plan a careful rotation in the veggie patch.