The white crosshairs do extend 40mils in each direction, whereas the yellow crosshair shows only 10mils. So there are situations where it is helpful to line up the two crosshairs and hold a more accurate lead beyond 10mils. This is particularly true in 5" guns firing at longer ranges on ships traveling perpendicular to your guns, as the correct lead can easily be around 20-30 mils in those cases.
You can also use the white crosshairs to determine the precise point at which a shore battery will stop/begin traversing. Again if you line up the white and yellow crosshairs (in Sea Mode, CTRL+Q) and then traverse the shore battery to the extreme right or left, the white crosshairs will stop moving at the point past which the gun cannot traverse. This can be useful for determining the exact moment a ship will come into firing range, and you can line up your shot on a target before it comes into range and fire on it the moment it is in range. Likewise, you can prioritize your targets (often between the CA or CV) based on how long the target may be in firing range before it exits.
Kind of a limited usefulness, but both of these tricks can help get a few extra rounds on target.
