Wearing a shoe is a little like wearing a plaster cast.
A natural bare hoof expands and spreads as it bears weight and contracts when it is lifted, in much the same way as our own bare feet do.
This acts as a shock absorber and also helps with circulating the blood supply to the foot.
When a shoe is applied to the foot it is done off the ground in the contracted phase. When the foot is then put to the ground and bears weight it is unable to expand.
Eventually this reduces the blood supply to the foot. You will have noticed that the feet of shod horses are always cool, or even cold. Barefoot horses have healthily warm feet.
Shod feet eventually contract, which can lead to pain at the back of the foot - often seen as horses having a 'toe first' landing. This puts considerable strain on the tendons and ligaments as well as structures higher in the body.
Shoes add considerably to the amount of concussion felt by the body - around 75% more in fact!
There is also a greater risk of injury to both horse and handler.
The horse can brush or overreach; kick or be kicked; and it really hurts when they stand on your foot!