When a projectile is rasped with a file, the process imparts what is similar to knurling on to its surface which fractionally bumps up its diameter in size by a variable number of thousandths of an inch.
It's done by rolling over the projectile with a file on a hard, smooth surface with enough pressure to impart the knurling. The bumps that are raised effectively increase the measurement of the projectile enough that according to xXxplosive, he can more quickly load the ball into a fouled bore without needing a patch.
In essence, some commercial muzzle loading bullet manufacturers do the same thing when they impart knurling on to their projectiles. It helps to insure that the projectile is slightly larger than the size of most bores by just enough so that the projectile will better stay in place by grasping the barrel wall and by helping it to fill the rifling grooves. Yet it will load easier then if it were a smooth sided projectile of the same diameter. And if the bore is fouled with powder then it will be able to grasp all that much better.
This can also be done with balls at home by simply using a file. a bastard file according to xXxplosive.
Even when using a patch, if a ball size feels too loose then rasping them is an easy way to help make them fit tighter without buying additional patches. And it could possibly help bullets & balls fit more snugly into revolver chambers too.