Every new business deserves a business plan (as does every existing business); not just because it needs investment or financing, but because business planning helps set directions, and helps prioritize and manage specific steps, dates, deadlines and responsibilities.
When a startup doesn’t have to develop a full formal plan to show outsiders, then it shouldn’t spend the extra effort polishing a formal printed plan; just do the plan as the first step of planning. Set it down so you can track what you expected as the real world changes.
One of the best uses of a business plan for a startup is to add up starting costs, including initial expenses, initial assets and the initial cash needed as working capital to support deficit spending from the start until the point that the business reaches monthly cash flow break-even.
Another good use of a startup plan is defining and communicating strategy, as in target market focus, why to buy, main marketing message and media. Even when you’re a single person, with nobody else involved, you want to get this written down so you can track results. We trick ourselves: Unless we write things down, as events roll out we forget what we had expected. That spoils the opportunity to modify and correct based on changed assumptions. There can’t be course corrections if there was no course set.
And yet another good use of a startup business plan is setting down who does what and who owns what before the business gets started. These things are easier to lay down in detail before there is money involved.