The Privy Council keeps paranoid types, well, paranoid. This is because it is possible, in a political-thriller sort of way, to imagine how the Queen could dismiss her ministers, "prorogue" (or discontinue) Parliament and rule through her Privy Council. Which is how we used to be run, and Her Majesty's Privy Council is a quaint leftover from those days.
Nowadays it has only ceremonial powers, but it can stop you getting hanged in some Caribbean territories, and has "purview" over royal chartered bodies such as universities. The majority of its activities are a harmless waste of Privy Counsellors' time, but in return they, usually important politicos, get to call themselves "Right Honourable".
They can talk to each other on "Privy Council" terms, which presumably is when they speak the truth: hence the wording of the Council's oath, which dates back to Tudor times, to "keep secret all matters ... revealed unto you, or that shall be treated of secretly in Council". Feeling paranoid now?