An idea for a great product is often the necessary starting point for a good business venture. But there's more to business than just great products, particularly if the product is something tangible, like a device of some sort (as opposed to an iPhone app). A tangible product needs to be manufactured, distributed, marketed, sold and probably serviced and even updated from time to time. And all those activities have to be supported by purchasing, logistics, finance, management and accounting.
A company writing a business plan needs to be realistic about all the things that have to be done to build a successful business, not just the product development. (This issue can be particularly problematic for engineers and other hard-science types.) It may be the driver, but it will have to be supported by other business activities. And a good business plan must reflect that.