Why does utnapishtim tell Gilgamesh the flood story and what point is he trying to make by telling him it?

Utnapishtim recounts the flood story in response to a question from Gilgamesh: How did you find eternal life?

Utnapishtim's point is that he was granted immortality due to unique circumstances that will not be repeated. It is therefore futile for Gilgamesh to seek eternal life.