Allowing a close fit and keeping the bullet alingined with the bore. This is part of squeezing all the accuracy out of the gun.
This is an old fashion notion that full seating testing outright debunks.
If your freebore is normal, it's 1/2thou to 1thou over cal, regardless of neck clearance.
Bullets themselves are restrained within this clearance, and there is nothing you can do with a free chambered round to change this.
That is, you can neither help nor hurt this attribute, even when shooting bananas with extreme runout.
And believe it or not, you cannot prove otherwise.
What does happen with bananas having runout exceeding clearances, is conditions of chambered pressure points.
Only then are shots provably thrown -from runout.
I remember probably 40yrs ago, a HOF BR shooter would simply toss any brass used with a thrown shot. A simple solution for THIS problem, which was more common then, as chambers were tighter while heavy FL sizing was employed due to extreme pressure loads. Little has changed, except BR shooters have opened clearances more & more over the decades.
Common clearances are high enough today that relatively awful runout still affects nothing.
This applies to hunting chambers just the same.
The bore alignment you really seek is a dynamic interfacing with the throat, as dialed into best through full seating testing.