Mostly, it was that there had to be at least one to fix things if there was an actual invasion. Never dealt with demons, personally, but some of my old coworkers had stories. Not a cause and effect, witch = demon invasion. More the only logical solution for why this place is still standing. :)

I'd apologize for dropping him in your collective lap, but it seems like people are enjoying poking and prodding him. At least he speaks English now?