I have asked myself this question multiple times.  The jester or fool was someone whose sole purpose was to entertain the royal court.  

The Mardi Gras celebrations are all about the royal courts of old, thus, the costumes of kings and queens as well as the formal ending marked by the “Meeting of the Courts”. And as with all royal courts in the world, there will be a jester to entertain the king on his low moments.

The court jester, who was also called the Court Fool, was used to balance out the four humours – sanguine, choleric, melancholia, and phlegmatic – that were then believed to influence an individual’s general health. This is the time when the adage about laughter being the best kind of medicine started, which the court jester was the primary provider.

In medieval times, the court jesters were usually mentally handicapped people who would have found no other means of employment were it not for their unwitting capability to amuse the court with their actions and behavior. In time, the court jesters slowly evolved to be esteemed members of the court able to insult the royals to some degree without facing punishment. One of the most famous was Archibald Armstrong,court jester to King James.

In many ways, Mardi Gras jesters retains the role of the court jesters of old. Thus, you will see a jester juggling balls, telling jokes and generally just behaving in goofy manners to bring on the laughs.