A business plan is simply the result of doing basic research like:
* estimating the size of the market * checking out the competition * estimating your costs * figuring out what drives your costs * figuring out just how much you need to sell, and at what price, to cover your costs * agreeing who's gonna do what when, roughly
and a number of other things..
A "business plan" is just a piece of paper. Business planning, on the other hand, is essential - unless you like to fly with your eyes closed.