When starting and growing your business there are two things to always keep in mind:
Being able to work in your business
Being able to work on your business
The first involves the operational aspects of the business such as designing the product/service, hiring talent, legal, operational management, figuring out shipping, and of course customer service and support.
The second focuses more on the aspects of Marketing. Being able to create a brand, market and promote your brand and offer, and drive leads back to you in order to convert them into customers.
Having a Marketing Plan and strategy to grow your business is absolutely imperative. Many businesses fail because they fail to market and promote, and immediately assume their idea or product will get customers and generate sales on its own. Sure you can get by without a marketing plan, but if your goal is to truly build a long term, sustainable and profitable business, then having a marketing plan, especially one geared towards growth is essential.