The jury system was made part of our Constitution because the Founders did not trust judges, but ever since the Constitution was adopted, there has been a struggle between judges and juries, with judges trying to confine juries to deciding the "facts" in a case, leaving the judges to decide the "law". In most criminal cases today, the constitutional and legal issues are not argued in the presence of the jury, and the jury does not get to read the legal briefs filed in the case. Judges try to intimidate juries into following their instructions, which are only advisory, and prosecute attorneys for contempt if they try to inform jurors of their right and power to decide the law as well as the "facts" in the case. In response, reformers are working to make sure jurors know their duty.
It is important to keep in mind that many cases involve legal issues that neither the judge nor any of the lawyers have encountered previously. The trial is then a learning experience for all involved, including the jury. If the judge needs to hear the legal arguments to learn enough to decide motions, then the jury needs to hear those same arguments to decide the verdict.