There is a significant role for judges in trials as well, especially on complex technical issues. It's up to the judge to decide all "matters of law", for examp

There is a significant role for judges in trials as well, especially on complex technical issues. It's up to the judge to decide all "matters of law", for example which conduct would (if it happened) violate a law and which wouldn't. A judge can even throw out the indictment entirely, finding a defendant innocent as a matter of law, if they determine that the conduct alleged isn't actually a violation of any laws, even if it did happen. In complex fraud cases indictments being thrown out (or overturned on appeal) by judges isn't all that uncommon either. Where it goes to a jury is when there's a dispute of fact rather than law: the judge has determined that whether the defendant violated the law or not depends on a fact disputed between the prosecution and the defense (e.g. whether the defendant actually did or didn't do something, or what the defendant's intent was). The jury then decides whether the facts support conviction, although even then they're typically guided by very specific instructions from the judge about what they would have to find in order to convict.