As a founder, you should write a business plan, whether or not you need third-party financing. A well thought-through business plan helps you to better understand the key levers of your future business, to identify risks and to make up your mind how you would cope with appearing risks.
In start-ups, financiers usually ask for a meaningful business plan. They pay particular attention to the skills, experience and personality of the founder or founding team. It is also important to them that the market is accurately captured and the business model is robustly defined. A third important aspect is to know how sensitively risks are perceived and how you want to react to the occurrence of risks.