Say for example the FD in the 744? I know it has to be coupled with the AP and I think it provides visual cues on the HSI as to where/how to fly the plane. But I am sure it does more than that. Thnx for any info.

If the FD is enabled without autopilot it allows manual control if a flight plan is loaded. I'm not sure if it has any other uses with or without a plan.

The FD is known as a command instrument because, like a VOR, it tells you what to do. Something like an AI, ADF or DME (for example) is not a command instrument because it just tells you the current situation, you must decide how to deal with it (which way to turn, how quickly to descend, how much control yoke pressure to exert, etc, etc).When flown manually (with the A/P armed but not engaged) The FD tells you exactly how much bank or +/- pitch to use at any one time to fly the best path according to the flight plan or autopilot settings.With the AP engaged, it shows the commands being input to the flight system by the AP.