From the Fire Alarm Engineering discussion group on LinkedIn

Question:   I would like to ask what is the reason or purpose of listing fire alarm systems and components. I don't need the procedure or the definition, that is spelled out in NFPA 72. But what is the reason to start with? My first assumption is that it is used because most fire alarm companies do not have the expertise or capabilities to determine if components (relays, etc.) are compatible with the control equipment and also that they meet the basic requirements spelled out in NFPA 72.
Answer: When a manufacturer approaches U.L. for a "Listing" to UL 864, they must also submit all devices that U.L. will test with that panel to ensure compatibility and proper function within the limits of the listings for the components that may be attached. The "Compatibility List" included in the Owner\Installation Manual for each system includes the part number\models of every device that was tested with the FACP by U.L., hence, they are "listed for the intended use", i.e., attachment to that make and model FACP.

Manufacturers are in the business of selling product, so they quite naturally tend to only pay U.L. for testing devices they manufacture for "listing" with their panels. Some peripheral manufacturers do just the opposite: They submit their devices for testing and compatibility listing with those FACPs they feel they can match up well with.

From an operational and liability standpoint, failure to use devices "listed" for use with any FACP opens up the seller or installer to great risk, should a device or system failure result in the loss of life or property. Criminal and\or civil liability may also be incurred by not following the manufacturers' rules and listings when installing a system.

Basically, you either follow the listing, or you pays your money and takes your chances.
Mike Agri - Engineered Systems
TC Life Safety