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.
That is a great thing to make sure you have in your business plan, that your business will be built around those real customer wants. Do not make callous assumptions, or statements like "I know what people want", "People are going to love this", and so on. Have you done your research? Do you really know that the people you will be targeting want your product / service, and crucially do they want it AT THE PRICE that you will be offering it at? Whilst confidence in your plan is fantastic, you must make sure that it does not lead you down a blind alley along a path that is not desired by your target market. Don't assume what customers want, do your research and make sure that is clear from the start in your business plan.