When starting a new business or thinking of expanding, having a well thought out and articulated business plan sets the foundation needed to help assure a successful adventure. A business plan is a key asset that is mandatory when seeking to raise financing as your plan will need to tell a compelling story that will convince a lender that you have put sufficient thought into your idea and have conducted sufficient research to substantiate the key assumptions in your plan. If an investor or lender becomes skeptical about your business plan assumptions, finds key elements missing or worse yet, finds errors with information that is not synchronized or is missing detail, then the credibility of your plan is significantly diminished and the likelihood of it being accepted is quite low.
A business plan is also your yardstick to measure your performance once the implementation phase of the plan kicks in. A plan serves as a blueprint of what you're intending to build and a measurement tool to see how you're performing versus your plan.
Above all, think of your business plan as a story book that aims to keep a reader engaged and motivates the reader to embrace what you are envisioning and "get on board".
In summary, it is needless to say that it is absolutely imperative that entrepreneurs invest the time and resources necessary to articulate their "story" by way of a comprehensive business plan that will help assure they start down a path that will lead to the realization of their business objectives.