What I think they think she gives them is:
1. Youth and vigor so McCain does not look too old and hidebound.
2. I do think allengreen is right about appealing to the evangelical Christians.
3. She wants to drill in the Arctic National Wildlife Refuge and she opposes endangered species status for polar bears. The first is a very popular Republican position that McCain has been on record saying he does not support so this gives the ticket cachet with anti-greenies.