Many property owners require residents to maintain a renters or liability insurance policy throughout the term of their lease.  This is designed to protect both you the resident (tenant) as well as the property owner (landlord) from disputes over legal liability for accidental damages caused by you to their apartment. 

For example, in the event  you accidently start a fire in your apartment that causes $25,000 in damage to the physical building, you are most likely legally responsible to the property owner for the cost to restore the apartment to it's original condition.  In addition, you can be held liable to any other resident of the community for damages to their personal property caused by you.

For these reasons, many property owners try to ensure that all residents have insurance to protect them and the residents of the community.  Being listed as an additional named insured, allows the property owner to be notified upon cancellation of your policy.  This practice is very common in commercial insurance policies.  In reality, most insurance companies providing renters insurance policies will not agree to list your property owner as an additional named insured.  This is because in the event of a lawsuit between you and the property owner, the insurance company would have the obligation to defend both parties against each other and to pay any sums either might be obligated to pay each other.

As a solution, many insurance companies will agree to name a property owner as a certificate holder which provides them the notice protection they are looking for but does not make them a direct insured under the policy.  This is a similar concept to what your bank requires on your auto loan to insure you maintain auto insurance.