The lightbar is used for alignment, much like a CDI for flying a localizer. The Satloc system enables very accurate spacing working the field, and enables flying a racetrack pattern. This makes for a more efficient field treatment, eliminates the cost of flaggers or flags, like we used to have. With Satloc or AgNav, it makes returning to a field in the same precise location a snap, and it enables an accurate record of what's been sprayed.
By putting the lightbar out in front, where one is looking when one flies, it eliminates any need to go heads down in the cockpit, and makes alignment and spacing much easier.