For as long as this country has had monarchs, the secret business of royalty has been a source of intense interest.
In medieval times, the king was the fount of all patronage. Noble factions sought to acquire information about the tastes of the sovereign in order to gain access to those benefits. Insider knowledge of Henry VIII's moods and sexual appetites enabled families like the Boleyns and the Seymours to profit handsomely. They pushed their comely daughters in the priapic king's direction as a means of enhancing their own wealth and status.
Later, Charles I's personal rule between 1629 and 1640, without parliamentary scrutiny or consultation, cemented him in the minds of his opponents as a tyrant and sowed the seeds of distrust that sprouted into the Civil War.
After the Restoration, politics was dominated by suspicion that the Catholic monarch, James II, planned to re-impose the old religion on a now fiercely Protestant England. Suspicions about James's dealings with Catholic France helped to prompt the Whig coup in 1688 that finally put an end to the calamitous Stuart dynasty.
Of course the monarchy no longer has any political power. The 19th-century journalist Walter Bagehot distinguished between the "efficient" parts of the British constitution (which get things done) and the "dignified" parts (which provide the symbolism). These days, monarchy is strictly restricted to the dignified part. The Royal Family must be above politics and provide a focal point for respect from all Britons.
But that does not undermine the argument for scrutiny and oversight of royal affairs. The monarchy needs to be seen as a dignified figurehead. Financial profligacy or improper behaviour by members of the Royal Family would bring the monarchy into disrepute and jeopardise its constitutional role. Statutory transparency (in the form of the Freedom of Information Act) would help to keep royals on the straight and narrow.
It would also enable us to be sure that unelected royals are not unduly interfering in government policy and thus blurring the separation of functions between the monarchy and the executive. We have a right to see Prince Charles's notorious "black spider" letters to ministers; indeed it might be seen as a constitutional imperative.