Flight Director switch??

Could somebody please explain to me in detail exactly what the FD switch does?
I have been flying jets for some time, fly on Vatsim for a VA and fly IFR.
And yet...... I cannot seem to see what this switch does except for put a nice little pink thing on the Attitude indcator??
Thank you
I have been flying jets for some time, fly on Vatsim for a VA and fly IFR.
And yet...... I cannot seem to see what this switch does except for put a nice little pink thing on the Attitude indcator??
Thank you