The Freak System allows you to get more shots from every fill-up, no matter what marker you shoot. By bore-matching your paintballs, you waste less air propelling them down your barrel. This means you can turn your regulator pressure down, or your velocity screw out making your marker softer on paint and easier on air.
A bore that is too tight compresses the ball increasing friction, and increasing the pressure/volume of air your paintball marker must use to push the paintball through the bore. The added friction of an under-bored barrel can also cause broken paint.
A bore that is too loose allows propellent gas to blow by the ball itself, increasing the pressure/volume of air your paintball marker must use to push the paintball down the barrel. This can also cause inconsistent velocity because the amount of air that escapes around the ball will be varied from shot to shot.
Finally, a properly bored barrel has just the slightest gap around the ball to minimize friction and wasted air. With a properly bored barrel you paintball marker uses the minimum amount of pressure/volume to accelerate the paintball while keeping friction to a minimum.
The wide array of available Freak Inserts lets you match whatever paintball your shooting, getting the most out of your paintball marker.