Why does the American airman give Sarah a torn parachute in the novel, Think of England?

The American airman gives Sarah a bundle wrapped in brown parcel paper that contains a torn parachute, which will provide enough silk for her to make a petticoat and two slips. At first she tries to give the bundle back, but the soldier insists that she keep it because "You . . . . Why, you're what we're fighting for!"