Historically, skins were made of real animal skins with short, tough hair. They were used to make forward progress by sliding on the snow. They are fixed under the skis to move forward: the bristles slide on the snow when you press on the ski while sliding forward, and prevent the ski from sliding backwards, because the bristles stand up and hold the ski when it tries to move backwards. Then, when the time comes to make your descent, you have to remove them to glide properly.
So they’re essential for use when ski touring.