A competition neck sizing die (like mine from Redding) does not work the brass except the neck, nor does it pull the neck from the inside like a sizing button. Assuming theremaining aspects of your handloading technique are up to par, they tend to provide more consistency from round to round. If you are using a full sizing die, you work the brass every round and, depending how close to max pressure you are running, that can shorten brass life. This is mainly for bolt guns, as semi-automatics need to be full-size resizing every round to maintain optimum feeding and chabering capabilities. Some match chambers also need to be full-size resized every time to avoid pressure problems.
There are several handloading sites that focus on the extreme minutia of loading techniques - this is not the site for that in depth discussion.
To add to what was said above, it only resizes the neck and not the whole case. This means you can only fire those rounds in the gun that you fired the brass in the first time since all chambers are slightly diffrent, firing in another rifle may cause pressure problems or even not be able to feed in the rifle.
I think that neck sizing is something best left to the extreme target/benchrest shooters.
I believe in full length sizing all brass.
But I also think that cases should not be excessively sized. You only want to set the shoulder back one or two thousanths from your fired chamber dimension.
Here is how I set my dies:
I fire some factory ammo. Then I use a Stoney Point headspace guage and a set of dial calipers. Using these tools, I measure the distance from the case head to the datum line of the shoulder. The datum line of the shoulder is the mid point of the shoulder.
Then I adjust my resizing die so that the shoulder is set back by one or two thousanths of an inch. The resulting sized cases are now full length sized, but they are not excessively sized.
The result is that the cases will easily chamber your rifle. The cases will also fit your chamber like a hand inside a glove. And your cases will last longer. Accuracy will also increase.
But you have to dedicate a set of dies to each rifle. I have several 6.5x55mm rifles and they vary by as much as 5 thousanths of an inch. I have two seperate die sets. One set that I use with my Swedish Mausers and another that I use with my Sako.