The need to have a plan varies with project size. There are many positive reasons to have a plan and only one negative reason: cost. Usually cost can be justified by you and your contractor both understanding the project's size, scope and details. When everyone involved understands the project clearly, the margin for error and misunderstood specifications are minimal, therefore the project can be costed out with a high degree of precision and priced according to "on paper" facts rather than another person's interpretation of your thoughts. This allows multiple contractors to have the same parameters to bid assuring an apples-to-apples bid scenario.
Having a plan brings to your project unbiased professionalism and experience in committing your ideas to scale on your site and assuring feasibility and forethought. Architects and designers alike take aspects of the project into account that the average person would not think of. These aspects include lighting, shading, local building codes, color coordination, drainage, traffic patterns, views, specific space needs, weather conditions, and future plans, just to name a few. When having a plan drawn for a project, think about adding future projects to it. An overall plan helps current projects tie in with future projects, assuring that no good money is spent after bad.