A few dollars is all you need for your prototype compared to the hundreds of dollars traditional prototyping would require. It offers the much needed flexibility in manufacturing. This is because with 3D printers you can perform multiple iterations to your design and also test out the different ideas you have without putting your company account at any sort of risk.
All this you can do and at the end of the day still get professional and durable parts. You get to use real PLA and ABS materials to get real parts that you can actually test and fit. They include living hinges, snap fits, clearance fits and dove tails among others.