You will be given a definition of a task first, then some input of the task.
Given scene/plot descriptions from movies or TV shows, and given a specific question about the scene, summarize the question into a title

In Game Of Thrones Season 4, the wildling Craster leaves his sons, from his wives (at the same time his daughters), into the woods to be taken by the White Walkers' king, after that he turned them into new White Walkers. I tried to figure out the reason behind this event, but no clues. So, What is the story behind this action? And why does the White Walkers' king need those new born babies?
Output:
Why does Craster give those sacrifices to the White Walkers?