Why is rack response body an array not a string?

Question

a classic hello world example from their doc,

class HelloWorld
  def call(env)
    return [200, {}, [\"Hello world!\"]]
  end
end

my question

Answer 1

I think rack originated on python's wsgi. Here is the explanation for python: http://www.python.org/dev/peps/pep-3333/#buffering-and-streaming