Traditional crop rotations involved winter and spring crops with a grass ley break or root crop. This gave a patchwork quilt landscape where game could always find cover and food. Modern rotations mostly comprise only winter crops like wheat and oilseed rape, so the land gets cultivated all at once immediately after harvest.
Within a few weeks a summer landscape can become little more than cultivated soil. Leaving some small areas of stubble is an easy way of providing an early winter refuge for field living animals. Further, where you want to provide a little extra game cover you can hand sow in some patches of mustard into the crop. This provides autumn cover before it is killed off by the first frosts.