My answer to Importance of a business plan to an entrepreneur?
Answer by Ken Larson:
How are you going to do it if you cannot plan it?
That is another way of phrasing your question, which gets to the heart of a business plan. Others will ask you that question as you seek their help.
A business plan is vital to not only establish your own confidence in an enterprise launch, but also to convince co-founders, bankers, industry partners who can support you that your road map will reach its destination.