Why Biden for VP?

Why, ultimately, did Barack Obama select Joe Biden as his running mate? I think we heard the answer last night, from the lips of Sarah Palin. She has repeated several times that John McCain is the only candidate who has fought for his country. Last night she was all over Joe Biden about his son in the National Guard, piling on the respect for him and his family. I think all other things being equal, this respect our country gives to people with military service, and Beau Biden's National Guard service, were what pushed Biden over the top and into the VP slot.