A cargo box is used for transporting goods on a vehicle. Cargo boxes provide storage room in addition to the room offered by a car's trunk and interior. If your trunk is already packed to overflowing, space in your car is limited due to a large number of passengers, or you simply have oddly shaped items that won't fit in your car, a cargo box presents a solution. Cargo boxes come in many different shapes and sizes, with some that are designed to store items as long as skis, and others designed to be narrow enough to accommodate roof-mounted bike racks.