In emergency situations, audio PA alone often isnt enough to convey emergency communications to those who need it. For example, in the recent tragic events at Marjory Stoneman Douglas High School in Parkland, Florida, one of the first acts done by the shooter was to pull the fire alarm, which caused many students and teachers to believe that there was a fire drill and leave their classrooms before a code red lockdown could be enacted. More recently, at Santa Fe High School in Texas, the fire alarm was pulled, causing students and staff to, believing there was a fire drill, go outside, where they began to hear gunshots.
Fire alarms, by design, are extremely loud the alarm could effectively nullify any attempt to communicate via audio PA. In addition, many types of emergencies could render a substantial portion of the victims at least temporarily deaf, also rendering audio communications futile.
To most effectively communicate during an emergency, audio/visual PA would provide both the audio announcement but also a text announcement that would reach people unable to hear the audio for any reason. In addition, text announcements can continue scrolling, preventing people from missing important information. Its also easier with text announcements to provide different instructions to different locations without the other messages interfering and the message getting muddled or misunderstood.
Even in situations where everyone is able to hear clearly, having additional sources of communication is essential. An audio announcement alone could result in missed information if people were not paying attention quickly enough. Also, any single source of communication could see people seeking corroboration from another source. Audio/Visual PA provides two sources of communication in one and ensures that messages and information reach people in an emergency even in extremely loud environments. Additionally, if all official emergency messages and drills utilize both audio and text, then staff could know immediately that something was wrong if one was issued by itself.