Jesters keep the balance of the force. Before I explain though, I wanted to talk a bit about this pic, 'cuz I think it's badass. I don't know who the artist is, but this is what I envision it to look like when Terces enters Malegaunt's tower (in book 4, still in progress). That's right. He goes in like a BOSS - but does he come out?
So in a monarchy, or any other hierarchical system, really, there are people at the top and people at the bottom. The jester sits comfortably somewhere in-between, in touch with the common man, but familiar enough with the ways of those at the top to point out their flaws.
In modern society, those at the top are the people with the power - Senators, Assemblymen, the President, Presidential appointees, and heads of our secretive paramilitary organizations. The jesters are people like Jon Stewart and John Oliver. Now typically jesters don't have much power, and their words are, as George R.R. Martin would say, "wind". But we've seen with the Net Neutrality issue, how powerful these public figures can actually be, when they rally the people to political causes. This keeps the balance of things, showing the powerful that they cannot just do whatever they want, because there will be backlash, whether it is rotten tomatoes thrown by the people, or a joke at the king's expense around the dinner table.