Chances are, either your target customer or your business model is going to change, forcing you to re-think your idea. Good prototypes allow you to leverage your existing technology and change directions as needed. The most important function of a prototype is to validate. Even good ideas fail. A prototype allows you to test your idea, to find out if people will actually pay for it.
Good prototypes generate feedback from the market for better or for worse.
It's better to kill a bad idea before you take the finished product to market then to waste valuable time and money in developing a full version no one will buy. The market will actually test and research your product for you. Oftentimes, it will provide the specifications for the next version, indicating what potential customers do and do not like and what features should be included the next time around. In addition to validation, a prototype gives you something to sell. It provides a demo when you're pitching your idea to venture capitalists and customers.