The public address systems are a medium that is used to distribute sound throughout a decided space. The size of equipment usually differs with the size of the venue where it has been going to be used.
In order to make it simple to understand, the bigger and high-output PA systems are used for commercial buildings, discotheques and concerts while the restaurants, schools or home parties would need small scale PA equipment and amplifier with fewer functions.
There are wide range of equipment that makes a single PA equipment work the best way. This includes amplifiers, microphones and speakers that are used according to the application and usage of the system. Besides that, there are other equipments like ceiling speakers and column speakers that are used according to the venue and organized event.
Thus, keeping in mind the type of event and venue of the event, the PA equipment is used. The speakers are not suited for the venue that needs microphone and vice-versa.
Source: http://portable-sound-system.blogspot.com/2017/12/a-brief-guide-on-public-address-systems.html