I guess the advantage is the material is cheap, there is less distortion of the part when case hardening, and the surface can be really hard. If the entire part was left that hard, one could over tighten the tool post and crack the holder.
I used Heat Treating Bone Charcoal. I have a container about 6 x 5 x 5 made of stainless with a loose floating lid that drops down to the level of the parts and charcoal. A layer charcoal about .5" is on the bottom, parts are placed in and more charcoal is poured in, then a final layer on top, then the floating lid. There is a screw socket welded to the end so when the furnace is opened up, a 12" steel rod with a wood handle can be screwed on to lift it out. Up to temp and soaked for an hour or so. A 5 gallon plastic bucket of water with some sheet metal on the bottom is readied by bubbling air in it for an hour or so. Get ready to splash some water, I did it outside. Welding gloves on, open the furnace, screw in the handle, bring the container to the bucket, leave the lid on, and dump everything into the water. After cool, temper to keep the surface from being too hard. I'm not sure if tempering is needed so this could use some more study. Save the old spent charcoal to make up room if you only need to do a bottom layer of parts. Only new charcoal is needed just around the parts, or you can mix old and new to reduce the coloring (never done it but heard about the mixing).
If anyone has some additional info I'd be glad to hear it as I have not done much of this.