Which, in turn, closes the predestination loop, and lets everyone return for Harry Potter and the Goblet of Fire.
As Hermione explains to Harry in the book, Professor Minerva McGonagall (Maggie Smith) warned her that horrific things could happen if one time travelled irresponsibly.
Though this was a loop of events that were always going to happen, there are still rules to follow.
As long as Harry and Hermione have accomplished their goals, and make it back to their starting position by the time they left, without running into their past selves, the timeline will presumably remain undamaged.