Proof of Concept models are usually crude in appearance, but show how a physical or electrical feature works. Purely functional, non appearance model-making can be an effective way to prove out a mechanical concept without investing in expensive components or extensive rapid prototyping. The intent is support evaluating the over-all engineering function and performance. While these models are not something that you would show an investor, they help you and the development team understand exactly how your product will function in reality. The object of modeling is to know everything about your product before it gets manufactured, and Proof of Concept models are a quick and cheap way to do it.