Transport of troops, equipment, and supplies to the front and back again.
Example: D-day - 1. C-47s painted with invasion stripes roar over France, and drop the paratroopers in to clear the way for the invasion.
Example: D-day + 5. C-47's fly over troop positions (at least when they could find them) and dropped food, ammo, and new weapons.
And so on...