What is the role of a PM?

There was a debate on project managers' role (whether engineered for success or victim of circumstance) other day in office.
During the debate and after a long time I heard an original metaphor on project manager within an Indian IT services context from Ramesh Gopalakrishnan.
"you are a milkman asked to deliver 25 litres of milk to a customer at 5 pm, you did not decide the quantity or the time, you do not know if the cow will give the needed milk, but you are accountable for the delivery"

Does it sound familiar?