Just to be clear, the models under discussion here are 3D computer generated models, or digital models, not physical models.
It is rarely economic to produce a physical model simply to demonstrate how an item will look. There are times when a complex design or unusual structural layout will drive the need for a mock up that proves the concept, and in turn gives a representation of the end product. However, this scenario is unusual and for most situations a computer generated 3D model provides the client with the best appreciation of how the finished article will look.
In addition, as initial designs often need further tweaks or adjustments during a process of engagement with the client, these adjustments are most easily achieved with a computer generated 3D model rather than a physical model.