"Human beings are free except when humanity needs them. Maybe humanity needs you. To do something. Maybe humanity needs me—to find out what you're good for. We might both do despicable things, Ender, but if humankind survives, then we were good tools."
Graff is explaining to Ender the philosophy behind everything they are doing. Although Ender does not know it at the time, this is the same reasoning that the adults will use to manipulate the children time and time again. Ender objects to this idea, because he believes that people are more than just tools, but nevertheless it is the pervading ideology of the I.F. throughout the book. This philosophy justifies doing terrible things in the name of humanity, and it also means that individuals will have to make awful sacrifices for their species.