Advantages:
  enhanced welding and great protection from decarburization. Wrought iron is a fiberous material (having never been totally molten during the smelting process) with a high percent of inclusions - typically silicon. It was welded and rewelded under high oxygen blasts to 'refine' it (and to reduce the carbon content to close to zero). The welding was done under huge power hammers - hence the material was repeatedly hammered  - or worked - or wrought, hence wrought iron. Due to its composition, the material can be worked at extremely high heats (~2800..3000F or white) and welds easily compared to mild steel.  By wrapping a billet in wrought iron, the center core is protected from the blast of the forge and the carbon content of the billet is better maintained.

Source:
    This material was originally put into service as bars on a road prison in Atlanta and dates to just before the Civil War. Due to a conversion of the structure into housing, the bars became surplus and I managed to acquire a ton or so. The bars are 1.25" in diameter and ~6' long. It has to be hammered into thin plates prior to use, so the 50 bars I have represent more than a life time supply.