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TWR - truing the receiver face (by lapping, cutting, etc) with the bore ensures the barrel is parallel with the bolt; this gives the bolt lugs the best chance of all making full contact. Most AR receivers are not tight enough to hold the barrel parallel with the bore just by snug fit of the receiver to the extension, with the odd exception of course.

That may or may not affect accuracy, and of course most ARs are built without truing.