PIilot assistance
In airline flying, there can be specific functions and duties for both pilots. If one pilot is ''hand flying'' the other pilot may set heading bugs, run the radio, copy clearances and a myriad of things.
AS for single pilot IFR...I've done it and without an autopilot to boot...You are busy and you either get good or die.
Jets are harder to hand fly at high altitude...and a little help isn't out of order.