You can’t really grow the same crop in the same field season after season (without fertiliser), because they’ll sap the specific nutrients they need from the soil. If you do that over and over eventually the soil wont have any food for that crop. Growing something different each season that takes different nutrients from the soil lets it recover the other ones. I don’t know how it recovers on its own, circle of life stuff probably. Modern farming can cheat by artificially replenishing the nutrients with fertiliser.