427 cobra is right on about the chamber length gauge, it's a handy little tool, at 5 dollars a piece per caliber diameter you have it is a worthwhile investment.
Especially for people struggling with flowback and carbon ring deposits with slow burning powders it is a worthwhile investment.
Using a trim length that is close to the chambers shoulder, instead of what you find in a manual, will help the brass seal the chamber faster and greatly reduce blowback into unsealed chamber clearances.
It should reduce throatwear some, and your SD can sometimes be improved a very minimal amount.
Another important thing to remember, with trimmers like the Giraud, Gracey, WFT, CTS engineering, PMA tool, Trim it etc is that they all index off of the case shoulder.
So FL sizing and sizing/headspace consistency is crucial if you want consistent trim lengths.
The same can really be said for the Wilson unit but it's case holder contacts the whole case body taper, and the adjustment at the head so it is a bit different in design.
The good things about them is they are generally fast and easy to use, and a case head out of square with the body will not affect the trim length.
Not that this normally is problem in most modern precision bolt rifles though.
As what you achieve with consistent trim lengths on the necks is more consistent neck tension, but you must be quite good a shot to notice the difference of 0.001 in trim length in the first place.