“A business exists to create customers” – Peter Drucker.
As a startup founder you need to work on a business plan only if it is absolutely necessary and helpful to get funding. Rather most investors would want to see traction. Unless you are a proven entrepreneur with multiple successful exits, the days of getting funding with just an idea is over long, long time back. Bootstrap your startup, get traction and then think about spending time to write a business plan when you are forced to do so. Until then, spend all your time getting customers to use your product.
Prominent VC Dave McClure has said recently that “Don’t write business plan, rather build a functional product that people are using” (here). If you don’t believe in his words, here is another proof from more than 40 years. Intel’s Moore jotted down first business plan so small you would think it is a tweet. Check out the image below.