Well-designed mass notifications allow teachers and administrators to receive instructions that differ from those sent to students and parents, whether for a snow delay or a campus emergency. The same delineation can keep parents informed of school activities and events, without overloading and annoying the school staff.
In the event of an active shooter or terrorist attack, mass notification systems allow for a dedicated system to communicate what is happening and what civilians should do. This reduces the demand on cellphone systems or landlines as people are less likely to make calls looking for information, and frees up those systems for emergency responders.
Mass notification systems can require a notification be acknowledged both to ensure the safety of the recipient and to ensure their responsibilities are carried out. Confirmation or lack thereof from a recipient can be used to trigger further attempts to contact or to trigger dispatch emergency response personnel.