If the barrel bore is .020" out of alignment with the OD (which is fairly normal for factory barrels) and the bushing is machined to tolerances - say +.005" over barrel diameter, then the bushing will be tilting the moderator by +.015" at the bushing end - which is obviously magnified considerably at the muzzle....leading to the potential of bullet strike and its consequences.....
The shoulder of the barrel and the shoulder of the thread on the moderator should be the only touching components (aside from the threads of course)
The bushing is to prevent tilting only.
If, when you thread your moderator onto the barrel thread its touching intermittently (with each revolution), then it needs checking by someone who knows what they're doing.
Used figures as examples only by the way