The game needs attention throughout the day which is time-consuming for the average human who has a job and money to pay for the game....
High command purpose is to balance the game for these guys who can offer money but no extra time. (just take ten seconds and think about the mechanics that provides, buildings queue, troops rally point. it automatically builds recruitment office, range fire modes)
When you need it: In scenarios of retreat or at passing by bombardment (fire on the move) .
How the mode works, you select a route and your units start to move. If an enemy unit inside range your unit will stop and fire a volley. If your units are not ready to fire they keep moving until they can fire again.
This can already be done manually so it has to be an HC mode.