You wouldn't want mice in your house - and it's the same for farmers and their barns. Farmers work hard to ensure that their barns are kept as clean as possible. Part of this process includes ensuring that rodents, like mice, do not get into the barn. Mice can chew through the barn's insulation and wiring and cause significant damage to the structures or possible barn fires. More importantly though, mice and rats can carry and spread diseases to the birds so they must be prevented from entering the barns.
Turkey farmers maintain traps, like these ones, around their property to ensure that rodents like rats or mice can't enter their facilities.