That is a great question, which I cannot answer. I look at things like the G100 from Gecko, or all these 'gaelil' or IMS 'motion controllers' on ebay, and really wonder what benefit they may have for us..
What I do understand is that many of them use RS232 to communicate using some command interface. I think some have modes for sending a pretty basic coordinate, and it goes there, and others are far more involved. To use one in a CNC machine, in most cases I think you have to write a program that can take GCODE or similar script, and convert it into commands for the specific device you are using..
For example, your gcode parser on the PC could read the gcode, conver the move commands or whatever and then send them via a custom VB or similar program, and send the device off on it's way. I think very powerfull systems can be created this way, and I bet many devices have libraries and examples pointing this way.
However, all of that is conjecture from reading about PLCs and other machine control systems on manufacturer websites..
To really get it figured, find some stuff on ebay, find the manufacturer, look for the manuals, even if you cannot afford the device, read the specs..
Then you will know more than me.