Castiel was searching for an artifact. Nothing major, just a witchs' tome. Honestly, Dean and Sam could've gotten it themselves. But it was easier to send the angel.

So now he was at the door of one of the Winchesters' hunting friends.

If they know who has the book why couldn't they just call and ask? he thought to himself. His hands were in his trench coat pockets, gripping an angel blade just in case. He'd never met this "friend" of the Winchesters and was prepared for the worst.