From an economic point of view, marketing can do one of two things.
First, it can increase the demand for a good or service. It does this by making more people want that good or service.
Second, it can make the demand for the good more inelastic. This means that people will still tend to buy it even when the price goes up. Marketing does this by making people think that there is no real substitute for the good -- that it is in some way unique.
You need to be aware of this so that you can try to guard against being manipulated into believing that the product is unique, for example.
The primary objective of marketing is to influence the consumer behavior in favour of the company engaging in the marketing activities. Companies undertake marketing to understand what kind of products will appeal to the consumers, to design and manufacture these products, to facilitate purchase and use of these products by making them easily available to the customer at competitive prices, and by communicating to the customers information about the product to influence their purchases in favour of company's products.
Understanding the role of marketing helps the consumers to understand how companies can influence their behavior so that they can avoid unhealthy manipulation of their behavior by companies. For example it might help a person realize how a company is creates an attractive false image of a product, appealing in subtle ways to emotions and desires of people, even when the product is incapable of delivering the value implied in the image created.