Alexander Hamilton wrote in The Federalist Papers that juries act as check against judicial despotism by preventing "arbitrary methods of prosecuting pretend offenses". One way that a government can control dissidents is to charge them with a multitude of minor crimes, crimes which, under normal circumstances, most people would not consider an issue.
For example, from 1981 to 2003 Washington state law required that you have and use a litter bag in your car.
RCW 70.93.100 The owner of any vehicle or watercraft who fails to keep and use a litter bag in his vehicle or watercraft shall be guilty of a violation of this sectionDidn't matter if you actually littered, you were required to have a bag in your car for litter.
Under jury nullification a jury of your peers could recognize that this was a ridiculous attempt by the government to either raise revenue or harass an otherwise innocent citizen and refuse to find the defendant guilty.*
Obviously this is a trivial example and a law that was undoubtedly rarely enforced, but the point remains that governments, being at least as fallible as individuals if not more so, are going to pass laws that are either immoral, unjust, or both. One of the purposes of a jury based system is to prevent (or at least diminish) the effect of such injustices.
Recently both prosecuting attorneys and judges got in an uproar in Washington, D.C. when the Fully Informed Jury Association passed out handbills in Judiciary Square, an area where many federal and district courts are located. Why would these attorneys and judges be upset unless this was somehow a threat to their power?
A little history. In addition to Alexander Hamilton's remarks in The Federalist Papers, in 1649 a jury refused to convict John Liburne for inciting rebellion, and again in 1670 William Penn was acquitted by a jury of his peers on charges of illegally preaching. The concept of jury nullification thus dates back almost five hundred years to the Common Law on which our legal system is based.
So why have you probably never heard of the concept? The idea of jury nullification has the power to rile judges who feel that they should be the determiner of law in their courtroom. Over the years lawyers and defendants have been found in contempt of court for daring to introduce ideas of law rather than evidence into their arguments, more recently activists in New Jersey and Florida have been charged with jury tampering for daring to educate people regarding the law. The judge as overlord of the law, the jury a computer to weigh the evidence and determine which side it falls on. Not the original intent, by any stretch of the imagination.**
It is a fact of life that governments are, on occasion, going to pass laws that the majority of people generally disagree with. It's the nature of the system. A vocal minority strongly behind a concept, pushes their proposal using whatever means necessary. They are heard, but the majority of folks, having a less strong opinion one way or the other, don't bother speaking up. Now a law is on the books that most people wouldn't agree with if asked. The concept of carrying a litter bag in your car all the time is probably one of those times. What now? How do you get rid of the law without having to spend a lot of time lobbying against some hypothetical opponents such as Big Litter Bag, the manufacturers association, or the environmentalist lobby, who think that the alternative to having a litter bag is throwing each piece of litter out your window onto the highway. You definitely don't have the money to fight them, you really don't have the time between your job and your family and whatever little recreation time you have to yourself, so you don't. You accept that it's a stupid law and you hope that no police officer ever gets a wild hair and starts pulling people over to check their litter bags. The law rarely gets repealed.
Enter jury nullification. Sitting on that jury are twelve people, ten of whom think it's ridiculous that someone is going to be sent to jail for not having a bag in their car specifically designated for litter. Out it goes.
*Unfortunately this type of crime has also been placed under the term "misdemeanor" or "infraction" and you have no right to a jury in most states.
**BTW If you really want to be off a jury, mention that you believe that it is the right of juries to nullify bad laws. The prosecuting attorney will do everything in his power to make sure that you are on the first train headed home.