Why should you know the access code? For the same reasons that you have a key to all units!
As the landlord, it is your responsibility to teach the tenant/tenants how to use the alarm system, and what to expect from it. Since you are ultimately responsible, you will be on the central stations list of first responders (they will contact you first then others on the list if an event occurs where the system annunciates).
Additional benefits that you get in having an alarm system in your rentals are