One of the points of advertising is to get you to recognize the brand, so that in the future when you shop for similar products you'll think of theirs as a familiar brand (even tough you may have no memory of ever having seen their ads).

Ads also often get you to associate positive feelings with the brand or product. Again, this might not result in directly buying the product right away, but next time you think of it, you'll be more likely to buy it than if you never saw the ad.