The purpose of advertising is defensive. It rarely helps to grow market share, but maintains it by ensuring that a brand stays within the buyer's consideration set and prevents a competitor from occupying the advertising opportunity. It helps to ensure that the brand is noticed on the shelf, and that it comes to mind in a buying situation. According to Byron Sharp, people don't actually think about brands all that much; therefore, it's important to ensure that a brand occupies mind-share. As well, advertising rebuilds and reinforces brand structures and prevents them from being occupied by a competitor. It works best when telling us things we already believe.
Sharp, Byron. How Brands Grow: What Marketers Don’t Know. Illustrated edition. Oxford ; New York: Oxford University Press, 2010.