They involve killing people and you gotta make sure you aren’t the one being killed which is easier to do if your enemy doesn’t knows your plans

Attackers get to choose where and when to attack. Defenders get to dig in and prepare defenses.

The attacker don’t want the defender to know where and when, because then they will just reinforce that area. Defenders don’t want the attackers to know where and when they are weak (eg troop rotation)