usefulness plc modules

plc is useful for device control, device control have device start condition (input) and and control results (output), only with external switch-off input plc's input through plc logic operation to get results, then through the output terminal to control the external relays and other components to control the boot device, stop and so on.
LC sub cpu module, and expansion modules. When the terminal is not enough that you CPU module when necessary to increase the input module plc.