Why would you use a model instead of an actual object?

Often times, the actual object is much more expensive to create. That's why many designers and engineers create a model to represent their ideas to a group. The model is much cheaper and faster to create, especially if it needs tweaking multiple times. Also, that model can have working parts - you could take it apart to show the inside/cross-section or have interactive sections that allow easier presentation of your idea. Those are things you can't accomplish with an actual object.