A brand new barrel from the factory has imperfections inside of it, just like there are imperfections in brand new car engines.
In a car engine, although there are machines that have extremely close tolerances, there are always slight burrs or other imperfections in the metal, and thus are not perfectly smooth. On a brand new engine, after your first oil change, you can sometimes see very very small metal shavings. As the pistons continue to go up and down, these burrs smooth out and the engine becomes "broken in." Prior to breaking in a new engine, you should never run it at high rpms because it could cause damage in the long run. The same principle, at least to some degree, applies to breaking in a new barrel.
The insides of barrels have burrs and imperfections too. Granted, sometimes you can't see them with the naked eye, but they are there. As such, once you shoot a bullet through it, you should clean it out and make sure that you don't compress the crud into the metal and cause further problems. Also, you should ALWAYS make sure that you do not cause the barrel to overheat. Again, if you overheat an engine, you know what happens - so don't let your barrels overheat either.
So to answer your question, MUST you break-in a barrel just like I said earlier? No, you don't have to, but it is a very safe alternative.
Zachary