The purpose of advertising is defensive. It rarely helps to grow market share but maintains it by 1) ensuring that a brand stays within the buyer's consideration set and 2) preventing a competitor from occupying the advertising opportunity. Advertising 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.
- Buyers eliminate most options unconsciously: Buyers do not rationally evaluate every available option but choose from among a subset of mentally available options.
Sharp, Byron. How Brands Grow: What Marketers Don’t Know. Illustrated edition. Oxford ; New York: Oxford University Press, 2010.