Tire covers......why?

over the years i have seen many tires with side wall cracking, and most of the time its only the outside face, and not the inner side, as far as agricultural tires even if they have major checking in the side walls, how fast are they really going, its a big difference blowing a tire at 65 MPH as apposed to loosing a tire plowing a field. I put tubes in one truck that the tires where leaking so bad through the side walls, but it was only a plow truck to clear the parking lot in the winter, so we really didn't worry about loosing a tire. Steering, tires and brakes are the most important things to keep you in control any one will cause major problems and a scary ride... tire covers do help..