The requests for production ask for things like the landlord's ledger showing how they arrived at the amount they claim is due, communications with you regarding the condition of the premises or whatever, receipts for repairs they claim, citations by the government inspectors, photographs, deeds, licenses, registrations, and more. What is needed in an individual case will vary, of course.
Since sometimes it's your word against theirs, the power of a request for production is often that you get the paperwork that swings the issue in your favor. These requests have to tailored to your individual situation, custom-drafted by the lawyer to get the proof you need to win at trial. Used in conjunction with the form interrogatories which ask them to identify certain documents, the request for production can require the landlord to give you copies of those documents, not just talk about them.
Discovery should be done at the earliest opportunity after the motion to quash is finally decided, because things can go quickly after that, such as the shortened hearing on the demurrer. There is a deadline by which all discovery has to be completed, and if the landlord's lawyer tries to stall you with evasive tactics, and you need to file a motion to compel, even that has a deadline to file which you may not meet if you delay in beginning discovery.