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.
For an online business, a detailed and in depth business plan is usually not necessary unless you are trying to combine your online business with a traditional business. For most online business startups, the detail involved with planning a traditional business is not required. However, it would still be beneficial to you if most of the topics were still covered, even if only briefly. Having a written plan in front of you will help you to focus on important aspects of the business.