What Are Those Squiggly Lines on CNN Actually For? Ratings?

Sam Boyd, writing for the American Prospect, has an excellent point about the viewer dials that CNN uses during the debate. All my wife and I could tell is that McCain supporters liked what McCain was saying, Obama supporters liked what Obama was saying, and independents liked everything:

CNN has seized on the visual power of dial-testing data -- the positions of each dial are aggregated and the resulting numbers are plotted as a line on a graph with the vertical axis representing how positive people feel and the horizontal line representing the time -- as a way to modernize the look of its debate coverage. "If you look at all my colleagues' coverage of the debate, it looks like it could have been done 25 years ago," explains CNN election-coverage producer David Bohrman, who made the decision to broadcast dial-testing results during the debate.

But, besides goosing CNN's ratings, what exactly is the point of broadcasting dial-testing results during a debate? (Both Fox and MSNBC use dial-testing groups for post-debate analysis but do not put the results on screen during the debate.) Skeptics of the network's use of dial-testing point to uncertainty over the meaning of the results, problems with the composition of focus groups, and the distorting effect of watching the debate with a dial in hand. CNN counters that displaying the results keeps viewers engaged and gives them a baseline against which to measure their reactions.