Writing a business plan is a necessary part of starting a new business, because all of the how-to-start-a-business books say that it is. Even the best business plan is little more than fiction. There is an alternative.
A concept plan helps to identify the guesses and estimates in the average business plan, and helps to eventually turn them into facts. The idea is to help discover the right business model and right product. Find a way to test your product before actual customers, so that you will have something factual to show to potential investors. If your product, or your whole business idea, neeeds major changes, it's better to find out now, instead of after several years of struggle and money losses.
When it comes to deciding on a purpose or vision for your company, please come up with something better than "the leading supplier of (your product)." What business or consumer problem does your product solve? Do some consumer research to find out what sort of actual problems your product can fix. Are you ready to sell this product, and remain passionate about it, for the next 20 years? What will the product look like? Doing a demo with an actual example of your product will help greatly. Who is your customer? Why should they buy from you, a start-up, instead of an established company? Who are your competitors?
It's never too early to start marketing. How will you make your money? There are many different ways to do it. What is your pricing strategy? How will your product actually be made? If overseas, how will it be shipped from there to here? What about all the other parts of running a business, like accounting, HR and funding?
This book is short, and it is excellent. Written by a Silicon Valley veteran, it is full of information for start-ups of any kind. This easily reaches the level of Must Read for entrepreneurs.