On bigger buildings, maybe within a complex with more than one building comprising many separate floors, an alarm in building 2 floor 5, would cause the alarm to sound in all parts.

This could result in the whole complex being evacuated, or maybe one alarm throughout would cause the whole building to evacuate at the same time putting undue pressure on escape routes with a chance that people will be injured in the crush.

With Voice Alarms when the alarm is raised an activated voice message will be heard in the area that needs to be immediately evacuated, and other areas at risk will be evacuated via the Voice Alarm  message as and when the need arises.