I see a business plan as the touchstone to test all major business decisions and and to ultimately help build the objectives of everyone in the organisation. It’s therefore more important for it to be simple and easy to communicate than it is to be precise and in great detail. I’m a firm believer in the one page plan and usually create it on a MindMap. As the model cascades through the business the finer details are added as each part of the organisation builds their supporting plans.
A key element of a good business plan is that creating it helps management teams discuss and agree plans and subsequently communicating the agreed document helps everyone track and implement those plans. Without a good business plan organisations of any size tend to operate in disjointed ways which results in increased costs and poor customer service.