The purpose of jury instructions is to that the jury can make sense of the evidence they have just heard. A jury instruction is a simplified explanation of the law that applies in a given situation, so that the significant evidence you presented helps them see why you should win. Unless there is an instruction that makes sense of that evidence, they think, "So what? the landlord wants them out and they should have left!" With the right instruction, the jury thinks, "Ah, so that's why the landlord was wrong in filing this lawsuit!" The instruction has to be based in the law that is researched and noted in the instruction, and be a fair, clear, and correct explanation of that law. It's not easy to do, but if it's important to your winning, it has to be done..
While there are standard jury instructions in a book called CACI, those instructions don't cover all situations, and it may be required in your case to have unique instructions drafted to cover the particular issues that are important in your case. For example, there is no CACI instruction on the requirement to give tenants of foreclosed landlords special notices, so that if you are a tenant being evicted by the bank due to your landlord losing the place in foreclosure, you will need a special jury instruction to cover that.
Some of these have already been drafted for other cases, so if they already exist, this charge does not apply, since they are included in the Jury Trial Docs process. For those which have to be researched for your case, and carefully worded, the charge here is per instruction, and you will be notified to the number of unit of this service you will need to pay for, once they are done.