"Plans are useless; planning is indispensable," according to Dwight D. Eisenhower, then Commander of the Allied Forces in Europe during WWII. Now, you may be in total agreement with the first part of that statement, but you are really not convinced of the truth of the second part.
Business plans are mostly about organizing, formalizing, and committing to a specific plan_of_actions. Such a document generally presents the objectives, strategies, analysis, and a detailed roadmap for implementation. Underlying a plan_of_action is comprehensive analysis of historic, current and proposed results all supported with assumptions. If anyone doubts the interest in business plans, a Google search returns more than 62 million and Amazon list more than 77꺜 titles concerning this subject.