Basically a reflector sight meant your reticle, ring and other marks remained "calibrated" regardless of head position.
This was essential because in real life you worked out range to target and hence whether the target was in convergence and how much deflection was needed using the ring and tick marks.
Of course the reflector sight is not that important for 90% of gamers because most people use icons to read off range and either shoot from the dead six or deflection shoot by guesswork, insight and "invoking the force". Then again if they can pull it off consistently good luck to them
