Concept models are invaluable in the product design process for verifying ergonomics, size or appearance. Based on your needs, schedule and budget, models can be rough foam or clay or extensive prototypes indistinguishable from the end product.
For transportation products, clay models are extremely valuable for evaluating surface forms and real world transitions. Shapes and forms can change drastically from a sketch or computer model to physical mass. Clay models can be quickly manipulated for fine-tuning, then scanned back into the virtual workspace.
Concept models from 3D CAD programs allow us to quickly work around mechanical space-claims. Renderings and physical prototypes can be created via 3D printer or 5 Axis CNC mills throughout the design process.