Callous and Corns develop from excess friction and rubbing on the foot. Corns normally develop on the upper joints of the toes, whilst callous generally develops under the sole of the foot (especially under the big toe/little toe joint and central ball of the foot). Corns and callous commonly develop when excess pronation disrupts the normal metatarsal bone relationship in the foot and this causes the toes to claw. Friction & rubbing from your footwear then causes corns and callous to develop on the tops of your toes and/or the bottom of your feet.
Orthotics control excess pronation and prevents dropping (plantarflexion) of the metatarsal bones and furthermore clawing of the toes. Thereby removing this friction and rubbing which causes corns and callous.