The NYT describes the strategic thinking behind Obama campaig efforts to expand the playing field a bit:
Mr. Obama’s aides said some states where they intend to campaign — like Georgia, Missouri, Montana and North Carolina — might ultimately be too red to turn blue. But the result of making an effort there could force Mr. McCain to spend money or send him to campaign in what should be safe ground, rather than using those resources in states like Ohio.
That's the normal account you hear, but I think in some ways it's the least-compelling reason to try and run a geographically broad campaign. The best reason, it seems to me, is probably just that it's an appealing electoral strategy to be seen as running a broad-based, nationwide political campaign. Bush talked in 2000 about the problems of poor minority children in school not so much because he thought he was going to get huge numbers of black people to vote for him, but to signal to voters everywhere that he was "a different kind of Republican," caring, etc. Even if Obama doesn't have any realistic prospect of winning North Carolina or Montana he certainly wants to win in places like Minnesota and Virginia and parts of Minnesota are like Montana, parts of Virginia are like North Carolina and an image as a broad-minded person who campaigns everywhere can be helpful. After all, Obama's eruption onto the national stage was a critique of the red/blue politics of cultural division, so it's good to dramatize that by running a nationwide campaign.
Beyond that, the more places you campaign the more places you're in a position to take advantage of unexpected good fortune. If for some reason McCain commits some kind of horrible gaffe that alienates the people of the big empty square states, it's good to have laid the groundwork to take advantage of that. Or maybe Bobb Barr will catch fire in the Deep South. In a narrowcast campaign, you need to guess in advance how things will unfold over the next several months and that's just difficult to do. If you have the cash to run a wide-focus campaign, then you can simply try to respond competently to events as they unfold however they unfold.