At it's heart, a deference of responsibility said when one seeks your guidance. Instead of sending them in the wrong direction, you tell the them the truth, that you can do no better than praying to the god of averages that your answer might be correct. No man knows everything, and this might just be a better world when we accept our own imperfections.

Questions where this is a legitimate answer:
What time is it?
Does God exist?
What's for dinner?