In the chapter, On Writing, King states that a writer should 'write what he knows'. What ideas justify this statement?

In On Writing, King claims a writer should write what he knows. He doesn't mean this in a literal way however. He feels it is important for a writer to have background knowledge of that which he is writing, but to allow creativity to flow around the story line. He gives an example using John Grisham, a lawyer, who wrote about his profession, law, but created situations that were clearly not realistic.