A business plan is nothing but a plan for your business. It is an articulation of your vision on how the future will play out.
A business plan also articulates how the startup proposes to go from point A to point B, and by when. It also outlines the milestones and other dynamics (costs, resources, revenues, etc.) on the way from point A to point B. I.e. It is a plan of how the concept of your startup will alter the market, and how you intend to implement that disruption.
But at startup stage, there is no past data that can be used to make reasonably dependable predictions. Hence the vision of what might happen in the market with your concept is based on assumptions that you have made based on your conviction and your insights. Even in more established companies, there is only so much predictability you can bring into a business plan based on past data. How in-market dynamics may change is an unknown, and business plans even of larger, established companies can and often do get disrupted.
Some of the assumptions you have made will play out as assumed, others will not. Nothing surprising about that. Why then is it important to make a business plan knowing that what happens in the market is most likely to be very different from what you planned for? Click here to read more.