Rally Points make it that as soon as a unit is trained, you can make it automatically move to a location whether you are on or not. Fire Control is when you tell your guns when to fire. You can make them stop whatever they are doing and fire, fire at enemies when they are stationary, fire when fired at, or don't fire at all. The only real benefit AI see from High Command is the queue. This makes it that you can have another building or unit start being produced immediately after the previous one if finished rather than needing to sit and wait until you return.
“Dad, how do soldiers killing each other solve the world's problems?”
― Bill Watterson, Calvin and Hobbes: Sunday Pages 1985-1995: An Exhibition Catalogue
"Rule 1, on page 1 of the book of war, is: 'Do not march on Moscow'… Rule 2 is: 'Do not go fighting with your land armies in China."
Bernard Law Montgomery, British general