Outside neck turning is done to match case necks to the tight tolerance of chambers with tight necks. Reaming the inside is a procedure done when necking a larger caliber down to a smaller one. Unless this dimension is reduced by inside neck reaming, this results in a case neck that is too thick to allow the bullet to be released when it is fired, causing pressure to sky rocket.
What is the difference between turning the outside of case necks and reaming the inside?
Sierra Bullets > FAQs > What is the difference between turning the outside of case necks and reaming the inside?