Starting a business without a business plan is like building a house without the proper foundation. Ask any architect and they will tell you that the higher the building, the deeper and wider the foundation of that building has to be. The foundation of the building has to be able to sustain the weight of that building - and anything else that may try to come against that building (wind, hurricanes, etc.). And so it is with building a business.
No architect comes up with a concept to build a structure and immediately starts building. No! They first draw up plans. The plans itself accounts for almost everything. The windows, the plumbing, doors, load-bearing beams, etc. They take the time to thoughtfully consider what resources will be needed to build this magnificent structure because they know that anything out of place can bring that structure crashing down.
When building an empire, the same thoughts and considerations must be given. This is where the business plan comes in. The business plan helps entrepreneurs account for resources needed, and any other variables that may come to threaten their empire.
A business plan includes the following areas at a minimum:
The mission, vision and purpose for which the business exists
The business' target market: This considers what demographic the business intended to reach?
Competition consideration: Who else is offering similar products or services? What threats does the competition pose to the business, if any? What competitive advantage does the business have in the market/industry they are in?
Human Resources considerations: Does the business require additional staff to fulfill its mission? What skills must the business owner and staff have to be able to effectively do their job?
External Resources: Consider what outside sources are needed such as additional vendors and supplies that will be needed on a regular basis. Will partnerships be required to successfully operate?
Marketing considerations: How ill the business get their products to their target market? How will the business gain customers and/or clients? How will the business communicate their value?
Business operation considerations: How will the business run? What will the business process be? Is it efficient?
Financial projection considerations: How much money will the business need to start and to continue to run the business? How much profit will the business stand to make?
With such important questions to be answered, it is not wise to start a business without considering the foundational aspects of what will sustain that business. Most of the risk and variables that will come up during the life of a business (at least at the very beginning stages) can be seen and minimized, if not averted by "drawing up the plans" for the business. Why not set yourself up for success?