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.
In many cases business plans are very important but so much of the time it's a plan to try to convince someone else that you know what you are doing with your business like banks, investors, partners, etc. Now it's true that a well written business plan can also be a major benefit to your success as well if done right. It can guide you and keep you on track and can be the vehicle to get you were you want to be especially with so many outside forces now days that bombard you. A plan can be extremely important to your success especially when you look at the statistics that says 51% of small businesses fail sometime during their first 5 years.