Le fou du roi: What he teaches us about the giving of strategic advice
It is in the nature of jesters to speak their minds when the mood takes them, regardless of the consequences. They are neither calculating nor circumspect, and this may account for the "foolishness" often ascribed to them. Jesters are also generally of inferior social and political status and are rarely in a position (and rarely inclined) to pose a power threat. They have little to gain by caution and little to lose by candor—apart from liberty, livelihood, and occasionally even life, which hardly seems to have been a deterrent. They are peripheral to the game of politics, and this can reassure a king that their words are unlikely to be geared to their own advancement. Jesters are not noted for flattery or fawning. The ruler can be isolated from his courtiers and ministers, who might conspire against him. The jester too can be an isolated and peripheral figure somehow detached from the intrigues of the court, and this enables him to act as a kind of confidant…
The foolishness of the jester, whether in his odd appearance or his levity, implies that he is not passing judgment from on high, and this may be less galling than the "holier than thou" corrective of an earnest adviser. One of the most effective techniques the jester uses to point out his master's folly is allowing him to see it for himself. Rather than contradicting the king, the jester will agree with a harebrained scheme so wholeheartedly that the suggestion is taken to a logical extreme, highlighting its stupidity. The king can then decide for himself that maybe it wasn't such a good idea after all.
The jester is in a sense on the side of the ruler. The relationship was often very close and amiable, and the jester was almost invariably a cherished rather than a tolerated presence. This leads to the kindliness of jesters: they could be biting in their attacks, but there is usually an undercurrent of good-heartedness and understanding to their words. If they talk the king out of slicing up some innocent, it is not only to save him from the king's wrath but also to save the king from himself - they can be the only ones who will tell him he suffers from moral halitosis."
Source: Beatrice Otto, Fools Are Everywhere: The Court Jester Around the World