In a gunfight, you usually have to expose yourself at least a little bit in order to aim and fire. And let's say that you know an enemy soldier is around some corner, unaware, and you can pop out and shoot them. If there is another soldier aiming at your position, unbeknownst to you, you are dead.
In WW2 most shooting was covering fire, not targeted shots. That means people where not aiming shots, but just firing in the general direction of the enemy. If the 80% would have done it, the positive would be the other 20% would have been much more effective with the only downside of increased ammo consumption.
