You're thinking of advertising rather than marketing as a whole. Think of marketing as the social component of design. You wouldn't start coding an applicatio

You're thinking of advertising rather than marketing as a whole. Think of marketing as the social component of design. You wouldn't start coding an application before you had at least a rough design of what its functionality would be, right? So why would you start designing a product before you identified who is going to use it and what they will be using it for? Marketing answers those questions. In rough, marketing identifies your market: Who will be using your product and why. Your application / business is then built using that knowledge.