"No feet, no horse" is a cliche but it is no less true for that. A poorly shod horse can be severely affected. Try walking around in wong-fitting shoes and see how it feels. Horses are no different. The shoe should always be made to fit the hoof and not the other way around. The rear of the hoof (the frog) acts as a pump for the whole circulation system so it is important that it is no constricted by a narrow-fitting shoe. The angle of the shoe is also crucial to ensure that the ligaments and tendons don't come under undue pressure in much the same way as high-heeled shoes in humans can cause all sorts of musculo-skeletal problems.