For long term maintenance of competitive advantage, a firm must ensure that its methods cannot be duplicated or imitated. This requires constant analysis and regular reinvention of competitive strategies.
If an online business, you need to go into detail how you will attract customers to your website. General statements like "I will use Face Book ads and email marketing" will contribute almost nothing to helping your cause unless you have detailed statistical analysis of tests you have conducted or of another similar business you have been associated with. If you do not have any data upon which you reference your estimates, it could show lack of proper thought to the remainder of your business plan.
You have to do research and you have to put your thought in to it. You have to follow a certain format that will put everything in perspective. Luckily enough, you can find business plans examples or even templates if you just know where to look.
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.