In order to sell a product or a service, a company must establish a relationship with the consumer. It must build trust and rapport. It must understand the customer's needs, and it must provide a product that delivers the promised benefits.
If you don't believe in your product, of if you're not consistent and regular in the way you promote it, the odds of succeeding go way down. The primary function of the marketing plan is to ensure that you have the resources and the wherewithal to do what it takes to make your product work.