This type of rug is mainly intended to protect your horse from draughts and cold when sweating after riding or lunging. Whether the horse sweats so much after work that it needs a rug depends on the intensity of the work. If you are only going at a relaxed pace through the forest, a sweat blanket is usually not necessary, but it is necessary after intensive jumping or dressage training.
In winter, horses that are not clipped sweat very quickly. Then you should always put on a sweat sheet after riding. You can also put the rug on your horse during the autumn to prevent it from developing a thick winter coat. It can also serve well as a transport rug, at least between spring and autumn. A sweat rug also keeps a sick horse warm and protects it from draughts.