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Not much except to insure that the fire-formed, unsized body of the case keeps the round aligned with the bore instead of dropping to the bottom of the chamber.
A full-length sized case cannot align the bullet with the bore and makes for a sloppy fit with the extractor often effecting the alignment also.
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Probably the most stated "myths" in reloading for accuracy.
Those believing this is true, take the bolt out of your rifle, twist the cocking piece so the firing pin falls, then look at the bolt head. There's one, sometimes two things that push the case head a few to several thousandths away from the bolt face such that the shoulder on a rimless bottleneck case mates perfectly in the chamber shoulder. That centers the case shoulder, neck and bullet very well in the bore regardless of how much clearance there is around the case to the chamber wall. There's something else that pushes the back end of the case against the chamber wall; up for some, to the left or right and sometime up a bit, too, on others.
While the extractor does effect alignment, it's effect is microscopic and one needs stuff that shoots sub 1/100th MOA to see its effect. Besides, it's 100% repeatable from shot to shot so its influence is masked by that repeatability
If you cannot figure out why such cases center perfectly in the chamber's front end, post your issues and I'll explain further. This is really easy to figure out; just pay attention to all those things that put pressure on the case head after the bolt's closed. After figuring all this out, you'll also learn why few, if any, rimless bottleneck cases rest on the bottom of the chamber after the bolt's closed and especially when they're fired.
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Furthermore, full-length sizing induces neck run-out.
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Only when the fired case neck is sized down way too much and the expander ball comes up through it; this usually bends case necks somewhat. Setting the expander ball to start up the case neck while most of the case neck's in the die helps keep it straight. Using bushing dies without expander balls eliminates this issue.
There are ways to physically measure case neck alignment with the bore on chambered rounds. It's interesting to watch the case neck center up perfectly when the round's chambered, even with brand spankin' new cases in oversize chambers. Especially watching a new .243 Win case neck perfectly center in a .308 Win chamber.