Marketing plan templates are developed by people who have achieved success online. They capture the lessons the author has learned along the way and save you the time, cost and effort required to develop your own. Once you have mastered a particular marketing plan approach, you can then expand by refining the template you have used or by adopting another template that might be more suitable for the marketing of a different product or service.
A good template will provided a step_by_step approach with explanations as to why each step is proposed. The strength of the marketing plan derives from the integration of each step. So if you leave out a step, you may undermine your own plan.