When a horse has been working hard and is very sweaty, cooling it down too fast can cause muscle aches, so a sweat scraper is used to remove the sweat from the horse's neck and body so that it does not chill. It is then rubbed down firmly with a wisp or cloth and walked to cool it down slowly.
Sweat scrapers are also used when washing a horse so that cold water does not sit on its coat and make it cold. After hosing, the sweat scraper is used so that the cloths used to dry the horse are not saturated immediately. Some people don't use cloths at all and removing the majority of the water stops the horse from becoming cold as it dries.