For a home-brew project it is a lot easier to use a PC as the controller than to have to design a dedicated controller. That's why people do it. But commercially we are talking the potential for a machine to last a number of years, more than a lot of PCs last. And the trouble with using a PC is that a few years later if something breaks it is likely you will have to replace the whole PC unit as parts are generally not readily available. Or something like that.