A Fire Detection and Alarm System (FDAS) is a system that detects fire and warns people of danger. As such, it should never be turned off; it's a life-safety system. If it's 120-volt detectors and pull stations connected to relays that sound bells throughout a building, it's still a life-safety fire alarm system. If it ever was turned off, people occupying a building would not be warned of a fire.
The system can be as simple as the firefighter's 120-volt AC outside horn which is connected directly to the waterflow switch, or it can be as complicated as a multi-panel system using 3,000 smoke detectors and thousands of horns and strobes. The power source for the fire alarm system needs to be protected so it doesn't get turned off.
Nowadays, the breaker panel needs to be in a secure area, so only authorized people can get to the panel. I have seen, in an unlocked panel in an apartment building's laundry room, where an unauthorized resident had turned off the circuit breaker for a fire alarm system. Not a good thing.
Even authorized people, doing building maintenance, can accidently turn off the wrong circuit breaker and forget to turn it back on. It's been known to happen. A breaker lock greatly reduces the chances of an accidently turned off circuit breaker.
It doesn't matter whether it's a low voltage control panel for a fire alarm system or a 120-volt relay-based system. The NFPA Code says that because it's a life-safety system, you are to protect the power source for the fire alarm system.