“Why would he intentionally hit him in the 4th game of the World Series when there’s already a guy on base?”
It sets up the double play with a slow Posada coming up and one out. Until the umpires put a stop to it by threatening to eject their pitcher, the Phillies won’t walk Arod intentionally — they’ll just hit him.
Or...they thought it’s easier to get A-Rod out by pithing inside and Blanton missed his spot. A-Roid is a big boy, he can take it.