If you are an inventor and do not have experience in manufacturing, the best approach is to obtain intellectual property protection and license the idea to a manufacturer. A good demonstration makes licensing much easier.
If you are an entrepreneur, you need a good demonstration to convince yourself that this is a good idea, and to show potential customers and consumers to gain their feedback and orders.
If you are a program manger, you need a good demonstration to show your colleagues in Sales, Manufacturing and other parts of the organization to obtain valuable feedback.
In all cases, you demonstrate it first to yourself. You make sure it works like you want it to. You come up with new ideas and ways to improve the idea. You get a sense of it and then show it to some of your closest advisors. They have feedback and more ideas. The product starts to take shape and you feel confident it can sell, or you might find that people are not very interested in your idea at all.