Juries exist as a buffer against the tyrrany of judges. In order to graduate from law school, you have to shed every ounce of common sense you have and think in a very mechanical way. And if you lift up a judge's robes, you'll almost always find a prosecutor underneath. Juries ensure that common sense manages to find its way into the courtroom.
Of course, there's the whole "one tyrant three thousand miles away versus three thousand tyrants one mile away" thing...