What exactly is the flight director?
The flight director allows a human pilot to fly like the autopilot would. It turns on HUD elements corresponding to cyclic and collective controls that, well, direct you how to fly.
Turning the flight director on also disengages most auto pilot functions but leaves damping active. It makes the helicopter fly more like a helicopter, which can be the worst or best thing in the world to you depending on opinion.
To me the most important reason to use the flight director is that the AP is not infallible, and chances are there will be a combat situation where it gets knocked out. It's always good to be ready to handle this crazy bird without the AP's help and turning the FD on is a quick way to approximate this situation. For "day to day" flying and tank busting I leave it off.