Re: 308 brass
The main reason for turning necks is to assure that the neck thickness is consistent in all cases. This would produce similar neck tension case to case which would translate into consistent bullet release. It is just one more method to get each piece of brass as uniform as possible for consistent pressure shot to shot.
If the chamber is not setup for neck turning, especially a factory chamber which is usually quite large, then the necks will get work hardened and will split prematurely. I suppose annealing on a frequent basis could prevent the problem.
If using a FL sizer with a fixed inner neck diameter then the turned necks might not get sized small enough to make proper neck tension. A bushing die would solve that problem.
It is possible to turn the necks just the right amount to be sized by a standard FL die and NOT use the sizer ball. It takes some time to setup the neck turning tool but you can establish a precise press fit between the sized brass and a seated bullet. I used .002". By doing this I didn't have to get a bushing die set. The sized brass has zero runout if the die is of good quality.