Comment by FatherOfCurses

In a jury system, the judge provides instructions to the jury about how they are supposed to weigh the evidence presented against the requirements of the law. They don't need to be technical experts. The theoretical idea of "a jury of one's peers" (while not perfect) is making a fresh decision based on the merits of the singular case, rather than a judge deciding based on prejudices they've built up over hundreds of previous cases.