The roofs of cars are pretty much just "there." In the winter, they serve to keep the driver and passengers dry and warm. In the summer, car roofs serve to keep the driver and passengers dry and cool. That is basically where the purposes and usefulness of car roofs end -- or is it?
Think space! There is a limited amount of space inside the passenger compartment of a car and even less space in the luggage compartment. Most of the time, the limited space isn't an issue. We drive our cars to work or to school. The merchandise we buy on a regular basis is easily accommodated in the space provided. We can transport groceries, laundry, and kids in the allotted spaces provided for passengers and luggage in our cars without a problem every day of the week. But then the weekend comes along every five days.
On the weekends we use the very same cars that we use during the week, but now the limited luggage space becomes a big problem. We decide we want to go camping. We need to take camping equipment, bicycles, etc. and the luggage compartment is just not going to do the trick. Now what?
The answer is fairly simple. There are roof boxes and roof racks (sometimes called cargo carriers or luggage racks) that can be purchased or even rented that will accommodate all of that extra "stuff" that you want to take with you on your weekend trips and outings. Likewise, if you are planning an extended car trip and your luggage compartment won't hold everything you need to take with you, roof boxes or cargo boxes are waterproof and provide a place that is just as safe as the luggage compartment in your car.