Our bodies are pretty good at communicating; calluses are hard areas of skin that often develop in response to pressure or friction. Calluses indicate where we tend to bear our weight. Callus formation is the body's way of protecting the foot's underlying tissue against excessive pressure; a built in defence mechanism. Pressure can be due to an underlying cause such as abnormal foot mechanics, including bunions or hammertoes, or caused by mechanical stress, such as poor fitting footwear.
Orthotics are very effective at addressing the underlying cause by redistributing pressure and increasing the surface area in contact with the foot. An effective orthotic transfers pressure away from the high pressure areas to areas where we can better tolerate the pressure reducing callus formation. Footwear with an extra wide toe box and extra depth can also be important in redistributing pressure. Keeping an eye on skin condition and callus formation is especially important for someone who is diabetic and more prone to developing infection.
Essentially they are a built in pressure map of our feet, showing us where we bear our pressure.