There are many reasons for utilizing such a document. Is the final document going to be about implementing a decision already having been reached or is it about analysis and recommendations for a newly proposed activity. As noted above, a finished document may be for internal or external purposes. Externally they are often used to solicit funding for a start_up or joint venture. Whatever the purpose, do not confuse effort with say, a marketing or a production plan.
The "Back of a Napkin" Plan: It is the source of entrepreneurial legend and lore, the million_dollar idea that was hurriedly scribbled on a bar napkin. Yet, for most potential business owners this option for business planning remains a fantasy. However, like any myth there is a tiny grain of truth inside. A quickie business outline can work as a launch plan under the right circumstances.