The Three-Eyed Raven says that Bran needs to leave and the time has come for Bran to take on his role.
For the past six seasons, the Starks have played a major part in the Game of Thrones narrative.
There are major spoilers about Game of Thrones Season 6, Episode 5 The Door.
This chiefly happened because Bran finally used his past-hopping capabilities to learn a hard truth and find out exactly where the White Walkers came from.