The periople lies just under the coronary band/hairline of the hoof capsule.
Its role is to protect the softer new horn that is being generated to grow new hoofwall.
The rest of the wall of the hoof is protected by a thin layer of tissue that protects it from outside environmental insults. (You can see just part of this covering on the hoof above – and can clearly see where the hoof protection has been rasped away.)
When this protective covering is damaged in some way, i.e., rasping the entire wall of the hoof, then these outside factors will damage the hoof.