Why would one want to remove the breech plug from a new barrel...on a 'new' build project?
1. To make sure the barrel maker fitted the plug properly (sometimes they don't).
2. Ease of inletting just the barrel into the stock, then inletting with the plug installed.
3. If a liner or drum is used, anything protruding into the bore must be removed/smoothed, so jag/patches don't hang up.
There are likely more reasons.
Why would one want to ever remove the plug, if the barrel was well taken care of?
1. To remove a stuck patch/ball.
There are likely more reasons.
-Ron