Some areas of the world you are required to register every firearm. A barrel assembly (as this) allows one to have the frame registered and still have different configurations or calibers.
That was the basis of the Omega system when Springfield Armory introduced here in the U.S. In Germany, the Omega was the Peters Stahl, and allowed different calibers for different competitions and still have one handgun.
"A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed."
Life Member NRA
Life Member NAHC
Former President of the ECPT (Eifel Combat Pistol Team)