You need a business plan if you're a business owner. A business plan is like a "road map" and a "compass" —without it you will get lost. The biggest business plan mistake is putting it off.
You definitely will need a business plan if you are going to apply for a business loan; need investors; have business partners; have a management team; or is selling the business. In other words, you need a business plan.
You can use your business plan as a tool to generate interest from financiers, prospective employees and strategic partners. Please remember, the business plan is basically an "argument" where you need to state the problem and pain, and then provide your solution with supporting data and analogies.