If you measure the brass thickness on the neck with your micrometer, you'll notice that there can be a difference between minimum and maximum. If the difference is a lot, then the bullet will not be centered in relation to the case and chamber. This effects accuracy.
A special neck turning tool is used to make the neck thickness uniform.
Not much to worry about in hunting ammo. Match ammo is another story.
Don't confuse neck turning with neck trimming. Trimming needs to be done, since the case grows in length after being fired and full length sized. After a few firings (and sizings) the necks may need to be trimmed.
The excess brass that causes the case length to increase comes from the area of the brass near the case head. As it gets thinner, the chance of a case head seperation increases. Can't just keep trimming the neck. Proper head space will lessen the amount of case stretch.
Don't worry about the difference in the Neck die vs. the FL die. The neck die doesn't need to size the body of the case or contact the shoulder. The FL die is precise and made to strick tolerances.
You can't just keep shooting and neck sizing. After a few firings, the brass will begin to lose it's elasticity. It won't spring back and allow for easy extraction from the chamber.
You'll have to FL size then. Better, IMO, to FL size all along, but just enough for easy chambering without contributing to the above mentioned case stretch.