Role of timer in fire alarm?

Depends. Most fire alarm panels have delay overrides in them so maybe when the fire alarm initially gets triggered, it waits a few minutes/seconds while the person investigates the cause of the alarm. Once the time is up, the alarm sounders/bells automatically start activating.

In some other cases, you could have a class change feature in which the panel is programmed to sound the fire alarms at a certain time in the day- for example, class change in a school, the fire alarms produce a signal from the panel to signal class change.