Moving back into a poise pattern as an opponent launches an attack

Why do combatants adopt a poise pattern at the start of combat? Certainly it is not to show that they practice kungfu (and then throw their kungfu patterns away when the combat starts). It is because adopting a poise pattern gives the combatants certain advantages. The poise pattern, for example, can keep an opponent at bay. When you move in to attack an opponent with a poise pattern, you have to open the way or close the opponent.