Voldemort was defeated, and the head of the snake was cut off, so to speak. But that didn’t stop the snake’s body from inflicting damage. The leftover Death Eaters were desperate, bloodthirsty radicals and very much active in Britain, Scotland, and the continent as a whole, and Hogwarts was a prime target for attacks. Harry didn’t know exactly what Hermione would be doing down in the Department of Mysteries, but he sensed it would be extremely helpful to the D.M.L.E. and their fight against dark magic.