I do not know the purpose of inside neck turnig. If the case neck diameter has been changed by necking up or down, there could be a minor variation in neck wall thickness. If uniform neck thickness & concentric bullet seating are the goal, then outer neck turning is the only way to achieve this. Using inner neck turning only follows the existing profile. While the neck wall might get thinner, resizing will only continue the variances in neck wall thickness & not provide uniform wall thickness or concentric neck / case alignment. If outer neck turning is done, then a pilot is used for centering the neck & only the surplus metal is removed from the outside of the neck. This provides uniform neck thickness & facilitates concentric seating of bullets in a neck that is aligned to the rest of the case.

Having said that, it probably makes little practical difference for medium or big bore hunting rounds that provide a minimum of 1.5" accuracy at 100 meters & are used up to a maximum of 300 meters.