Until the 1980s, logic control (which gives a machine its sequential operations) was usually created by wiring lots of relays together in series and parallel making thousands of wire terminations causing reliability problems. It was awkward to make circuit modifications, expensive if the system needed lots of timers and counters and you needed a massive panel to keep it all in.
So Programmable Logic Controllers were developed to replace all of the relays, thousands of wire connections, rows of DIN rail and big panels.
It was all done using microprocessors,and opto-couplers but the smart bit really was the visual interface, you wrote the program in a format already very familiar to the electrician ... Ladder Diagram. So it was fairly easy (with training) to convert your knowledge of hard-wired relay circuits into using PLC based systems.
If you’re involved with fault finding on automated systems you really need to be able to go online with the PLC and view the logic operations and the timers and counters of the machine in real time, otherwise you’re working “blind”. So the vast majority of engineers taking up PLC courses are not going to be programmers, in fact they might never alter a program in their lives but they will use their knowledge of the PLC system as the best fault finding tool they could have.
Google to search for a Training Provider or have a look at my website for ideas www.training-industry.co.uk