You need to itemize your budget with breakdowns for advertising and promotions. Include any design, development for needed materials to facilitate the marketing. List the media outlets that will be used to execute your marketing plan and break out the online, social, e_mail marketing, e_brochures/e_catalogs, TV, print, radio, PR, tradeshows, sponsorships, philanthropic partnerships, direct_mail, telemarketing, and other outlets. I would also identify which components that will be completed in house and whether you will have an outside agency to help with your media buying and planning.
Imagine if you can do all these and manage all these in a marketing plan? Systematic, organized, well thought, innovative and captured all your pertinent business information, don't you find having a marketing plan is useful for your business?