Why is the closing scene in the Alchemist important

Because of its ambiguity. It seems that Santiago has finally come to the end of his coming-of-age journey. However, his last line implies that he has one more thing to finish - and it will require another journey. He needs to find Fatima, after having found his inner truth. In this way, the author conveys an idea that journeys never actually stop, and the goals are not at the end of the journeys, but in the journeys themselves.