Because different plants have different root characteristics and require more or less weeding and cultivation through the season, where and when you grow them affects the weed population in that section of the garden. Thus, crops that don’t compete well against weeds, like corn or carrots or onions, might well be planted in the plot where potatoes or squashes were the year before. Because the squashes and potatoes were likely subject to frequent shallow cultivation (and, in the case of the potatoes, hilling), annual weeds will be less vigorous in that area of the garden. Also, by alternating shallow-rooted plants like cabbages or lettuce with deep-rooted plants like tomatoes or turnips, we allow the plants’ roots to do much of the soil loosening that would otherwise have to be done by hand.
One last effect of this type is the traditional allelopathic relationships between succeeding and preceding crops. That’s a fancy way of saying that some plants leave behind residues that will help or hinder other particular plant families. The classic example is the standard corn-soybean rotation of the American Midwest; both crops help the other beyond the level of just their different nutrient needs. Some gardeners feel that beets and carrots hinder following crop and are themselves hindered by following a legume such as beans or peas. There is an enormous volume and variety of such information around—some of it mere hearsay, some of it well documented. Gathering such information and testing it in your own garden can be an interesting and very informative project.