Forgive me if I botch some of the details, but a few decades back Coca-Cola conducted an experiment where they tracked, in excruciating detail, all coke sales in a small town for a month. Then they turned all the ads off. All the television ads, newspaper ads, all the signs and and all the billboards. Coke sales immediately dropped ~30%.
Everyone knows what Coca-Cola is, and yet they have one of the largest advertising budgets in the world. You think that split second decision at the restaurant last week where you asked for a Coke instead of a water, a beer, or a Pepsi was of your own volition, but it very well may have been because you drove past a coke billboard on your way to work that morning, or because you saw a commercial a couple weeks ago.
Everyone thinks advertising doesn't work on them, but human psychology is tremendously complex and chock-full of easily exploitable bugs.