The most common reason I towed trucks from the back was rear end damage severe enough that the rear wheels couldn't be used. There's no way to front tow a truck that has the rear axle ripped loose from the frame for instance. Depending on the situation, a suitable flat bed was not always available or needed, and even if one was, it would still often require having a stinger or boom lift truck on scene anyway, to assist with loading the downed vehicle by lifting the rear up. A front tow was always preferred of course, just not always possible.
