Sometimes bullets do not always fit the gun they are fired in, there can be variations (differences in headspace). These variations cause inconsistency in accuracy.
So once you fire a bullet and the powder starts to burn that case's shoulder and neck will expand because it is brass and seal to the chamber of the gun. Reloading that case will yield you a custom cartridge that will have a headspace better matched to the chamber of that gun. Whenever there is a space in-between the case and the chamber the bullet can behave erratically, because powder might not burn evenly and input uneven forces on the bullet causing it to wobble.
Generally you can reload a higher quality and more accurate bullet than what can be produced in a factory at scale.
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