Coolers are made for cooling out and drying a hot/sweaty horse or a horse that has just been bathed so that they don't catch a chill. Typically, I only use these in the cooler weather.
During the winter months, prior to a show, they will have their winter blankets on, so that will help to keep them clean. After a class, clinic, lesson etc. we will put a cooler on them to help dry them out and keep them from catching a chill.