The idea behind the cards in O.O. is in a way to simulate fog of war and c&c challenges. You as the 500foot tall general might know that the best option for squad A is to lay down covering fire for squad B to advance, but your squad commanders might not even be aware that the other squad is in a position to provide support.
Basically there are cards to represent the most common actions a unit would want to take: off the top of my head the cards are move, fast move, ambush, suppression fire, fire, and assault. If you don’t have an appropriate card to assign an order to a unit, they cannot execute that action: rationalise it how you want (communications breakdown, squad leader missing an opportunity, etc), but it makes sense: soldiers are not robots, they will not always know the information that the commanding officer knows, and they make mistakes.
The cards also make the sequence of actions ever-changing: each card has a number determining when that action will be executed, which makes it more difficult to meta-game. Instead of “My infantry move at 6″ per turn. Well, that open field looks to be about 18″ across, so they should be able to cross before the enemy LMG team gets into position”, you end up in a situation of not knowing what priority cards your opponent is holding, and being forced to balance your own objectives against the priority and order cards in your hand.
Those are brave men knocking at our door. Let's go kill them!