Why do brand names accompany punch lines?

When simply watching TV commercials or even when thinking about the creative process behind making those commercials, it seems intuitive to build them up towards a punch line and to present the brand name (and/or the product) together with this punch line. Still, it keeps me baffled just to see how often this is almost the only brand name mention in some of these ads. From a strategic rather than a creative point of view, such practices are pretty much like stone age wisdom applied. Granted, TV might not be the most sexy and up-to-date advertising medium according to some of the advertising innovators, but stone age...? Or am I missing something and is there a vast amount of research out there contesting some of the old school wisdom?