Keeping Schtum

"Thou shalt not grass" is the First Commandment among the criminal classes, as it is, albeit more elegantly expressed, among boys at expensive schools. In Sicily they call it Omerta.
There are countless situations in society where sealed lips are mandatory. The priest in the confessional, the solicitor with his client, the doctor with his patient are everyday examples. Much official business is protected by secrecy, as are the doings of gentlemen's clubs, Freemasons and, for all I know, the Flat Earth Society. Judicial discussions between members of a bench must never be revealed to anyone.
I have attended many meetings where we have accepted Chatham House Rules, which allow free discussion, but oblige participants not to repeat others' words outside. Top of the tree are Privy Councillors who are bound to keep confidence about all of the many secrets that are revealed to them; this allows for uninhibited debate among people who might be opponents in a different context.
So all of this helps to oil the wheels, and makes for sensible governance - so long as it is not abused to deceive the public.
That's why it is such a shame that the death of Blair Peach led to a closing of police ranks that resulted in a successful conspiracy to defeat justice. Don Corleone would be proud of them if they were his soldiers.