I don't think all advertising is about informing you of a product, some of it is about getting something on your mind and making you want it. Everyone knows what McDonald's is and everyone recognizes CocaCola, but showing you hamburgers and fries or pouring out a Coke in front of you gets you to think about it and start craving it. Of course, Ian Sane has a point about seeing a McDonald's commercial after he's already had it, but that's more a matter of timing adverts well than adverts themselves.
Plus, companies spend millions researching the effectiveness of adverts, and if they didn't actually help their business/profits then I'd think they'd figure it out.