Similar to a dimmer light switch, 100V PA system volumes can also be controlled using an actuator, if the controls are not part of your centre. If, however, you want to achieve additional flexibility, for example to render constantly returning to the PA centre from whatever room you happen to be in at the time unnecessary, you may want to include one or several of these devices. They can be installed on-wall or flush.
Note: the total output of the speakers to be controlled must not exceed the power of the actuator!
Mixers make sense once you wish to transmit different programmes through separate amps, or if you want to control different zones (rooms / speaker groups) comfortably.
This, of course, in addition to the mixer's classic use: bringing together and adjusting the sound from various sources, e.g. mics, CD players, or other audio devices.
With a proper PA centre, such as the Inter-M PA 4000 A, you wouldn't require an additional mixer, as it features an integrated mixer module.
One more example for your consideration: