Two reasons: There were occasions where the caliber of an older arm was changed. Ex. thousands of .58 caliber Springfield muskets were left after the Civil War. They were converted to a Trapdoor at .50 caliber The bore would be drilled out if needed and a sleeve of the new caliber would be installed inside the original barrel along with the Trapdoor. Another was once the rifling (grooves) inside the barrel wore down it became a shotgun and the bullet would not be spinning when fired through the barrel and accuracy would be lost.