I believe (noob here myself) that the pros use the stand ground setting on units and that allows a few advantages:
– units do not attack unless told to, meaning they won’t run under castles/defenses when you aren’t looking.
– units won’t start attacking whatever they collide with, meaning you can focus specific targets even when the enemy has a meat shield
– apparently you can shift click to send units to attack in a specific order
Some cons for us noobs though are that they won’t be effective in combat unless we are microing them. These intricacies raise the skill cap of the game, and is generally seen as a hallmark of AoE2.