I am a news guy, not an ad guy. I have, however, sold ads, designed ads, composed ads and even delivered ads at one point or another in my newspapering career. The one thing I've never done is studied ads, so what I'm about to say may seem terribly naive to my academic friends.
But it seems to me that there are two things you can do with advertising. You can create demand. And you can channel demand to a preferred resolution. Some advertising may do both, but they're really different functions.