Every farm should have a resident cat, unless it raises animals that could easily fall prey to one (modern chickens and geese can fight off cats well enough to protect their young, but anything smaller or less muscular, like ducks, would eventually start vanishing). The purpose of any real farming operation is to grow, harvest, and store large amounts of nutritious food, and nearly every vermin from single-celled funguses upwards will do its damndest to slip in and devour as much of it as it can so it can breed out of control. Worse, most have evolved as defense mechanisms methods to spoil the leftover food so other creatures will not be drawn to it and try to kill the vermin or steal the food for themselves (especially the humans that labored to grow and gather it). Cats are the domesticated answer to the vermin too large or too biologically similar to humans to be easily poisoned without the risk of poisoning the food as well, though their habits and instincts often anthropomorphize more negatively than those of dogs.