There are possible motions in limine before any particular witness goes on the stand, usually on evidentiary issues, or issues that may have been suppressed prior to trial, to ensure that the attorneys abide by the pre-trial admissibility rulings and does not inquire about X or Y. But yeah, this guy is supposed to be able to rule on these motions on the march. And even if his law clerks can and will do most of the heavy lifting and research, making a prompt decision requires some deeper knowledge or trial experience. Unlike Baseball, there are no Bench Coaches for Federal District Court Judges.