What happens when you finished the map of codex pages

Using your eagle vision, you have to arrange and rotate the pages so that it forms a map of the world. Once you get all the pages lined up correctly, you will no longer be able to rotate any of the pages.
Completing the mission will bring the standard rewards such as more health.
It also unlocks additional story and the next and final mission.

See 5:30 in the related video.