i worked as a technician for a japanese company that shall remain undisclosed but anyway in the machines in their assembly line i saw on a machine that it had more than one plc(programmable logic controller) in it, as well as a computer but with no monitor (machine just interfaced to the computer via a port) and i dont know if it had any microcontroller chips in it.. but anyway i have a few questions

why would a control system need all of these? why cant one control the whole thing? why more than one plc? what can a plc do that a micro or a microcontroller or microprocessor can't? and vice-versa? all i know is that a plc is usually more expensive than a microcontroller so what exactly are you getting for that extra you're spending?