Why do it? What will he air in that half hour? A few half-baked theories, from a political non-expert:
* A massive pretaped ad—along the line of The Man from Hope—to try to close the deal / familiarize himself to last-minute deciders
* Allows him the possibility of a Jeremiah Wright-type live speech to address [William Ayers, negative campaign charge yet to be determined… or Jeremiah Wright yet again]
* Assuming the country is still in financial crisis, allows the possibility of a fireside-chat-style address—to lay out plans, soother nerves, establish self as calming leader, hence sealing the deal, etc.
* More broadly, a speech—any speech—because recent history (Berlin, Wright, the DNC) shows that whenever he gives a major speech, focused on by the mass media, his numbers bounce up
* Just to show he can. Seriously. Much of the media talk about the airtime buy has been what it says about Obama’s (apparently) massive fundraising advantages, which feeds into a media narrative of inevitability. (Note to campaign: place call to Hillary Clinton, re possible drawbacks to media narrative of inevitability.)