It applies because the PCs *shouldn't* get the benefit of re-planning a detailed, elaborate combat strategy after contact with the enemy. Rolling initiative once per combat allows them the ability to plan and act instead of react. Knowing that you go after the fighter but before the rogue affords you the ability to retroactively pre-plan your actions, while in the heat of battle.
Players may scout ahead, discover enemies in a dungeon room, and before entering combat, sit in the next room and develop a plan of action. Now, they enter into combat. With a single fixed order of initiative, players have the benefit of *knowing* who goes next every round. This allows them to change their tactics, not as if in the heat of a chaotic 6 seconds worth of activity, but as if they were back in the other room, re-working their draft plan into Rev 1.1, then Rev 1.5, then Rev 2.
Instead of reacting to their enemies' actions in battle, and changing their tactics, rolling initiative once allows them to change their overall strategy.