1. Her purpose is to highlight Obama's lack of executive experience and rekindle concerns about his association with a radical church. Once she has served this purpose some revelation or event beyond McCain's control will pull her from the race, allowing him to gracefully select a more qualified running mate.

2. The oil industry wants her out of Alaska because she has cleaned up some favored kinds of corruption there. They knew she'd jump at this chance. Either they'll win, and she is out of Alaska, or they'll loose because of her, ensuring enough bad coverage of her that she won't be re-elected in her home state.

3. Her purpose is to discredit and marginalize the evangelical wing of the Republican party, who will rally behind her and then be blamed when she drags down McCain's campaign.

(I don't believe any of these but I find them entertaining. The interesting thing about the first one is that there was clearly a baby revelation in the offing and the National Enquirer is claiming to have been involved in a premature delivery of it. The last one is the most far fetched. The idea of open bristling at the evangelicals within the Republican party is hard to imagine. Got any more?)