As I continue to focus my work on The-Plan-As-You-Go business plan, I see, every day, how many people don't realize that the business plan that helps you run your business isn't necessarily a formal written business plan document.
It can be as simple as a clear understanding of strategy, as in what sets you apart, what you do for your customers, what you do particularly well, and how you focus on what you do well where that relates to what your customer needs. You can say it in 60 seconds as an elevator speech or build it into a presentation, or write it down on a computer keyboard. Or share it with team members often enough that you all know what it is.
Then you add action points, as in trackable dates and deadlines and responsibilities, and a sales forecast, and you have a plan. From there you start tracking results, improving things as results suprise you, and you've got planning going, which will help you manage.
When people who know say you don't need a business plan to start a business, or to run a business, and then they advise you not to have one, what they usually mean is that you don't have to have a formal written plan. They don't really mean you shouldn't be planning your business.