Prototyping and Testing
Construction of a physical model is necessary since a mathematical model will never be as complete and accurate as the physical representation. Prototypes are often very expensive but are the cheapest way to build the design without building the entire device. Testing of the model may range from simply actuating it to attaching extensive instrumentation to accurately measure physical properties and other parameters.
Example
For the example being used, a prototype was not the final goal. However, for this phase, a working prototype is constructed to ensure the design works. Adjustments may have to be made for unexpected last minute problems that arise. Testing is needed for the optimization of the final product.