I am currently undecided in the US presidential election. I would have voted for Ms. Clinton, given the chance, but that hasn't worked out to be an option. As a result, a lot of things might affect my vote.
Choice of veep is one of them. I'm no fan of either Mr. Obama or Mr. McCain; I'm not subject to the fantasy that the vice president they pick tells anything important about how they will govern, or that the veep will significantly affect policy during the administration. What it will tell me is whether either of them wants my vote. They don't really need *my* vote--it's only the one vote after all. But do they care about whether they appeal to me as a candidate? Will they suck up to me? That matters to me.
They have to know I exist to suck up to me, and that information will ultimately inform their decisionmaking somehow.