1) Isolated enemy troops: They can still do damage on supply routes, wreak havoc whenever allied troops landed there, and, if they had a way, travel back home to become reinforcements for the enemies.
2) longer supply routes: As land in between the major islands (that the US have taken) is still teeming with enemies, it would mean that ships and airplanes would not be able to be as safe, or make a landfall to get more freshwater or other supplies on board.
