Most Americans probably don't associate jury duty with the fundamental protection of our great democracy, but it's likely that the Founding Fathers had that thought in mind. In the Bill of Rights, juries are mentions in three separate amendments, more than any other matter focused on by the early States. In fact, President Thomas Jefferson once remarked that that if he had to chose between having representatives in Congress or representatives in the jury pool, he would choose the jury over Congress.
In our nation, juries were originally protected as the right of the People to participate in the judicial branch of government. For the first half of our nation's existence, the guarantee that all crimes will be tried by jury (Constitution, Article 3) ensured that both the State and the criminally accused had the right to trial by jury. Although today a criminal defendant has the option to waive a jury trial and have the case heard by a judge only, before 1930 a defendant could not waive the jury, because it was considered the right of the People to be part of the process.
Early in our history, juries began to fulfill their role as protectors of citizen democracy. Before the American Revolution, the British Crown recognized the dangers of local juries and transferred many criminal cases involving the colonists back to England (something that the Founding Fathers would prohibit by the Sixth Amendment). Like the militiamen who fought the Red Coats, jurors would later be seen as the populist representatives who could stand up to appointed judges.
Also like the untrained minute man who left his field and picked up a rifle to defend his country, the juror is an average citizen who may one day work in a factory, and the next sit in a jury box to ensure justice is administered. While federal prosecutors are on the government payrolls as members of the Executive branch, and federal judges are appointed for life, jurors serve a temporary role that brings common sense into the courtroom. The regular participation and rotation of jurors also ensures that average citizens have the opportunity to monitor the actions of prosecutors or judges, keeping everyone mindful of the citizenry which they are called to serve.
While it may be easy to bemoan a summons for jury duty, the juror plays a vital role in the functions of our American democracy. And, for most people, it is probably the most easily accessible way to protect other citizens from the possible tyranny of an unjust government.