The most important moments in a trial are often not seen by a jury. That is because it's one of the judge's main responsibilities to screen what they see and hear, lest they be prejudiced. It's the "you can't unring a bell" theory; once the jury hears something they shouldn't have heard, th trial is forever tainted. If the damage is great enough, a mistrial is the result. Judges basically prefer nuclear war to mistrials.