I think backplotting is like reverse engineering. You normally start with a visual representation of your toolpaths in a CAM program. Your CAM then posts the paths as a G-Code.
A backplotter works in reverse. You feed a program like NCPlot a G-code file and it visually recreates your toolpaths so you can verify that they are correct.
I think this really comes in handy when the G-code is hand written.