Harsh coated terriers such as Border, Airdale and Wire Fox Terriers have double coats. Each hair folicle supports a single hard outer hair and several soft fine liners that form the supportive undercoat. The characteristic rich colour and hard texture is formed by the outer hairs. As these grow out they become old, thin, soft and faded at the roots.
The purpose of hand stripping is to remove the old faded hairs so that the new growth can occur. If the coat is not refreshed it can lose colour and the hair folicles can become plugged up resulting in pimples or acne.