Typically when a hole is punched in sheet material, a slug of punched-out material becomes separated from the sheet. The slugs have no use whatsoever and are nothing but scrap. The slugs must be collected and either recycled or disposed of in some manner; otherwise they become a hazard to personnel and surrounding machinery. When the punching process is upward through the sheet material, the slugs often bounce around on top of the sheet material and are even more difficult to deal with.
USPTO Backgrounds
USPTO Backgrounds