Joad looked at him with drooped eyes, and then he laughed. "Why, you're the preacher. You're the preacher. I jus' passed a recollection about you to a guy not an hour ago."
"I was a preacher," said the man seriously.
(John Steinbeck, The Grapes of Wrath)
There is the before preacher in the former sentence, but a in the latter. To what end are different articles used?