I know the answer with respect to the jury instructions. If the jury gets a written copy of the judge's instructions (which are carefully wrangled by parties before the jury deliberation begins), the jurors will be tempted to suss out for themselves what the various parts of it mean. While the jury is the final authority on whether the defendant is guilty or not guilty according to the law, the judge remains the final authority on how the law should be read. (However, it is usually permitted to have printed copies of any relevant statutes.) It's the judge's duty to "charge" the jury correctly and correct errors in interpretation that arise. This requires those questions to be brought to the judge, often incident to a re-hearing of the relevant portions of the instruction.
When the jury has questions, and when the judge entertains them and possibly issues revisions or clarifications, all this has to happen in the presence of all parties, in the courtroom, so that the parties may object to any errors the judge may commit and thereby preserve them for appeal. I've had a case go all the way to a state supreme court based on the trial judge's error in the jury charge, so this is no laughing matter. (Except that the error in question was on the appropriate introduction of the "but for" standard for liability, and that invokes the couplet: "What's a 'but for?'" "Pooping, silly.")
I want to add that one of my most cherished old books is a pamphlet printed in 1693: The Charge Of the Right Honourable Henry, Earl of Warrington, to the Grand Jury at the Quarter Sessions Held for the County of Chester, On the 25th Day of April, 1693. It's a 28-page quarto, and indeed comprises the written instructions of His Lordship to the grand jury. This seems no longer to be commonplace.
