So everyone is aware, if you do not type an attack in your combat moves, then the attack never happened. I will assume pilots, naval officers and soldiers are incredibly lazy and/or cowards and will refuse to go without written orders. (So if you don’t include everything you want in the attack, you only have what you said you are bringing.)
In the case of not including everything you want in the attack, it could effect scramble orders, so this is to protect your opponent really.
In the care of failing to include all your battles, this decision is so I have something stated that I can point back to when I make a ruling. It might not come up too often, and generally if it does, the two of you can work out the problem without having to call in for a ruling (or at least I hope you can!). In most cases, I am sure your opponent will be willing to allow an added combat move if it is relatively harmless. (Forgetting to type an attack on an undefend transport, or to clear a destroyer blocking your allies or whatever.)