Boone led John around Sydney Airport, and warned him that it was up to him to save another survivor's life. Locke saw Charlie and Claire. Boone responded they were "fine... for a while" (hinting at Desmond's later premonition of Charlie's imminent death.) Locke saw Jin and Sun arguing, but Boone assured him that Sayid would lead them to safety, and that Hurley and Desmond would be fine as well. After John saw Jack, Kate, and Sawyer being accosted by Henry, Boone told him, he must clean up his own mess before he could help them. Boone, realizing John wouldn't comprehend on his own, led Locke to the top of the escalator. There, John found Eko's stick, covered with blood. Boone, now pale and bloody, as he was when he died, told Locke to save Eko before it was too late. ("Further Instructions")
Some info about Boone's purpose.