The RCBS Hand Case Neck Turner is designed to "peel" the brass from the outside of your case necks to a consistent thickness.
Most commercially available brass has variances in the case neck thickness. This leads to case neck concentricity and bullet run-out issues. Truing up the case neck produces more consistent and accurate hand loaded ammunition.
The cutter portion uses calibre specific pilot/mandrels (sold separately to the Hand Case Neck Turner) to guide the case neck.