The horse was lying on four different reasons: for fun; to maintain itself, and self-care; to relieve pain and discomfort; for rest and relaxation. It relieves irritation from sweat and gear, dry hair, or can be cooled, protected and relieves itching from insect bites.
This grooming during molting or from sweating under the sunlight. The horse will roll, if possible, after washing. Thus she tries to bring the skin to its normal condition, as when removed wash the natural fatty protective layer with different minerals.
Also, the horse relieves stress and muscle tension after work or long standing, stretching the muscles and vertebrae of the back and neck. Be sure to give the horse fell on safe sites. Don’t deprive her of one of her few pleasures that are available to sport horse!