Matthews 15 lb Empty Water Repellant Sandbag is cordura nylon, some are vinyl covered and waterproof. Sandbags are used whenever you need extra weight to prevent tipping. You usually "bag" stands, especially when the weight is off center, such as with a heavy Scrim. They help stabilize equipment in windy situations. The empty bag is completely sewn except for a 1" seam, left open for the user to fill and then sew closed. However, the user will require a heavy duty machine to sew the sandbag closed properly.