Dustyhands is right. Originally figging was used on animals to get them to hold their tails high. Supposedly in Victorian times the ginger root was adopted for young spankee's to keep them from clinching their bottom in anticipation of the next swat.
When you clench your butt cheeks you also clinch your sphincter. If you were to have an object in there that makes it more uncomfortable to tighten your cheeks, the idea is you are less likely to do so. So you have to choose between the pain from the spank or the increased burning sensation from clenching down on your ginger root plug. Leave it to the Victorian's to come up with something like that!
They were wonderful pervs.