The left/right bias is so you have as much "throw" to the left as you do to the right. Basicly it's like those cheap r/c cars that have the trim tab on the bottom for the steering. On those if you have that trim too far to one side you have awesome turning one way but will be taking a wide turn the other way. The purpose was in case the car didn't track straight, You could trim out the car manually and then use the trim tabs to fine tune it from the controller if it even has trim tabs.

So, If you have the gyro sorted out but your turns are wide one way or overly sensitive in one direction you can manual change the "center" stick to where you have equal throws. This would be much easier to see if the tail rotor was variable powered in both directions(or was two motors and two tail blades counter rotating) because then you would see that one side would have more power than the other. I hope that makes sense.