I'm constantly amazed by how many entrepreneurs just jump in and start spending money on websites, marketing, etc., without having first accomplished their homework. Then after several months and maxed-out credit cards, they turn to someone like myself to help them understand why they are not getting any leads or generating any business. In almost every case, these fledgling millionaires failed to complete a business plan before leaping into the unforgiving world of commerce.
A business plan forces one to apply logic and rationale to "bright ideas." Done right, it highlights the risks, tests assumptions, helps to identify winning strategies (if there is one), and presents the expected return on investment in cold hard numbers for different scenarios.
Completing a business plan before launching a business will not guarantee success, but it will give you a much better chance of succeeding. Rushing to market without a business plan almost always results in expensive surprises and ultimate failure.
Having a business plan provides a targeted roadmap and benchmarks against which progress can be measured. It also alerts you to "bailout" opportunities if things are not going as expected. And it will help you to avoid costly pitfalls. In short, preparing a rigorous business plan upfront will more than pay for itself.