Requirements in rule books and code books are what you can get away with. In essence, they say that you leave enough to do reasonable service to the device. That requirement can usually be met with a few extra inches of wire inside a back-box.
What you're talking about isn't shown directly in any rule book or code book. The service loop that you have observed is really a "Plan-Ahead Loop".
Planning for "In Case the Device Has to Move"
Often times, a smoke detector has to be relocated. The relocation could be just a few inches or fractions of a meter away, to quite a few feet or several meters away. Having to move the smoke detector doesn't happen very often, but it does happen.
As an example, see:
When a New Air Diffuser is Installed, Why is the Fire Alarm Company at Fault?
The requirement to move could happen at the time of certification of the system when the fire marshal sees a better place, or it could be years down the road when the owner performs minor remodeling. It doesn't matter why the smoke detector has to be moved, there are only about three choices to how the detector is moved:
- The wires are stretched - there's a limit to how far they can be stretched
- Install a legal junction box - takes time, and sometimes it's hard to get to a good place to mount it
- Use the extra service loop that a considerate installer left, just in case - that's the easy way
Most of the time, the wires for the detectors can have these service loops without impacting the performance of the system. However, NAC wires going to the Notification Appliances like horns, strobes, chimes, etc. can't handle the extra wire lengths.
Adding 10 feet or three meters to the service loop for each of 10 devices means nearly 200 feet or nearly 60 meters of added wire in the circuit. That's 10 feet to and 10 feet from each of the ten devices - 200 feet or about 60 meters.
With that much extra wire, don't count on the last few devices to work, especially during a power blackout. With a NAC circuit, the wire length was included in the design, so any extra wire may prevent devices from working.
The extra service loop on fire alarm input devices you see is a "plan-ahead loop". Planning for changes to the system makes changes easier. Just don't install that kind of service loop on output devices like devices on the NAC circuit.