In Summary : Planning out your business on paper first gives you long-term benefits with potential investors, employees, vendors, and suppliers. The business plan becomes your roadmap to success, with pertinent data that shapes the course of your business start-up and lets you adjust your journey as contingencies arise. Business planning templates are readily available and data sources abound at your fingertips. You will achieve a solid understanding of your business as you work through each section of your plan.
But your idea may be so new that you may also need to talk to potential customers, host some focus groups, talk to an ad agency, or maybe even make a prototype and float it past some people. Be prepared to spend the time. Remember, it's not about the Plan but the Planning.
You may find that the easiest part is the actual writing of the plan. The real work comes in the data-gathering, which may take you a hundred hours or more, depending on what you already know or have researched. If your new venture is in an area where you've been working, you may already know about your customers, your suppliers, your marketing plan, your organizational structure, your financial and cash flow needs, equipment, inventory, and so on.
"Plans are useless; planning is indispensable," according to Dwight D. Eisenhower, then Commander of the Allied Forces in Europe during WWII. Now, you may be in total agreement with the first part of that statement, but you are really not convinced of the truth of the second part.