I use PLC in some of my industrial panels. I use them to monitor digital and analog sensors, trigger equipment startup/shutdown, emergency shutdowns, control pumps, and much more. We don't really do any robotics. Mainly I use them when I don't want to install a dozen relays in a panel where one PLC will do it all. Basically, I use a PLC to monitor several conditions and if all is well, it will allow the equipment to run. Oh, and with the right add-on modules we can access them remotely if there is a problem. For some of them we connect them to a touch screen panel.
Do you already have a PLC? I primarily use IDEC and Siemens (which are basically the same thing).