The reason it is Cool is because if you know you are going to fight then it's about not giving the game away.
Think of every western you've seen where there is an ambush. The hero looks around, and catches those shifty eyes, the shaky hand, and that one bead of sweat and they know there is trouble.
Many gamers tend to see characters as perfectly logical machines that are only limited by their attributes. Using Cool is an acknowledgement of the human factor. Discipline is a great skill for keeping your head together, but the skill that determines whether you can appear calm to others is Cool.
