The requirements for conduit use for fire alarm systems are a lot of common sense "Mandates" in the NFPA Code. Keep in mind, though, that the NFPA Code, by itself, has no authority to "Mandate" anything, the government points to the NFPA published book and says, "Do what that book says to do".
"That book" is a series of books written and published by the non-profit organization National Fire Protection Association. The book numbers "70" and "72" are just two books from the long series. There are codes for fire sprinkler systems, fire suppression systems, elevator systems, just to name a few.
Starting out with the NFPA 70 Code (National Electrical Code or NEC), most of the wiring requirements for conduit are with Class III wiring. With Class III wiring, being power limited wiring, it's allowed to be the rules shown in NFPA 70, which is the National Electrical Code (NEC), and also in the NFPA 72, which is specifically fire alarm systems.
In both of these, some of the Class III wiring, or fire alarm wiring, can be installed without conduit. If the wiring is to be installed in some instances, like in hazardous environments, mechanical rooms, or other places that can harm the wiring, the fire alarm wiring needs to be protected in conduit.
But there are also other considerations. If there are common practices, like "everyone does it" that require more stringent installation, it might be a good idea to follow the practices.
More important, though, is what an Authority Having Jurisdiction (AHJ) or the government has to say. If their requirements are greater than what the NFPA says in the minimum requirement, according to the NFPA, those more stringent requirements take precedence.
In essence, the NFPA "Mandates" that you go by the greater requirements: what is written down in the NFPA Code, or what the AHJ says to do. Keep in mind that the fire marshal represents the government. If your government requires you to put the fire alarm wiring inside of conduit, then according to the NFPA in their definitions of the Authority Having Jurisdiction (AHJ), putting the wiring inside conduit is a requirement of the NFPA.