This is especially important when working in a team with others. The exercise of preparing a plan will cause the team to work through the issues and produce a common vision and strategy - one that they all subscribe to. This is very important if they are the ones to carry it out. Many times I have found that, after doing this as a team, certain team members may have very different ideas and goals for the business. If these are in conflict with one another, the business may be doomed from the start.
Some people advocate the use of a consultant to write a plan.
It may be OK to use such a person to assist in the process, but the CEO and the team which is going to implement and follow the plan should also be the ones to prepare it. Worry about the CEO first. If a plan is needed to attract a suitable CEO, it would be better to just prepare a simple overview document describing the product and market opportunity without the same level of detail. The Business Plan is a management tool used by well-run companies of all sizes and at all stages of growth to set down business objectives and goals and the details as to how these can be achieved. These details answer the who, what, when and why questions about the company.