You have designed a new product and wish to build it. It may be rather expensive to build a prototype especially if it fails, so you would build a small scale working model to continue further testing to make sure your design is practical and efficient. Eg: When a new aircraft or land vehicle is designed, a small scale model will be made for wind tunnel testing to ensure the aerodynamics work as per the designed intention.
In short, design it, build it cheap and make sure it works and then you can begin building the prototype.
