Outside neck turning is best for custom chambers, where you want a neck that is both thinner and concentric with the chamber. The gunsmith should tell you what neck thickness you want.
For regular chambers, it is best used very lightly to remove the very thickest neck sections and not to try to bring the whole neck into concentricity with the chamber. Best to use expensive match cases or to simply ignore the whole thing as a waste of time for a standard chamber.
Inside neck turning, or reaming, is required if you get a "donut" at the neck/shoulder junction.
Not necessarily. If you use an expansion mandrel after the brass is sized the donut is pushed to the outside where it gets turned off.
