After an entire year off-screen, the sixth season of Game of Thrones has been a game-changer for Bran Stark, the second-most neglected of Ned and Catelyn's brood. Bran has found new relevance to the story's plot with the aid of his greensight vision, a sort of psychic power that has allowed him — and the viewer — to uncover important truths from Westeros' past. Among those earth-shattering truths uncovered were the origin of the White Walkers and his father's not-so-honorable defeat of renowned swordsman Ser Arthur Dayne.
However, Bran's curiosity quickly got the better of him, and use of his new power came at a deadly cost — most notably, the loss of his faithful servant, Hodor. In a pinch, as the wights and White Walkers stormed the Three-Eyed Raven's cave, Bran used his warging ability — a form of mind-control — to coerce Hodor into sacrificing himself in order save Bran and Meera Reed.
Of course, that wouldn't have happened if not for Bran's interaction with the Night's King, who grabbed his arm and marked him during a vision. This allowed the White Walkers to not only locate Bran, but also to break the vaguely defined magic barrier that prevented the undead from entering the cave he and his friends were staying in.
Clearly, Bran made a huge mistake — and its toll has left a considerable bodycount already — but it might not be the worst of his "accidents" by the season's end. As some fans have theorized, he could similarly become the catalyst for the Wall's destruction in the season finale.
We last left Bran in the hands of, well, Coldhands — who was revealed to be his long-lost uncle, Benjen Stark. But unless Bran plans to stay with his uncle Beyond the Wall, which wouldn't make any narrative sense, he's going to have to return to Castle Black.
Therefore, if he returns to Castle Black and passes through the Wall, the same effect from the mark that destroyed the Three-Eyed Raven's cave may rid the Wall of its magic. After all, it was initially created centuries ago with (again, vaguely defined) magic in order to prevent the White Walkers from entering the Realm.
This would make narrative sense. The White Walkers have been an ominous, looming presence in the series from the prologue of the show, but eventually they'll need to become a tangible threat to the rest of Westeros. If they're stuck Beyond the Wall for the entire series, that point is moot. Even the highly acclaimed "Battle of the Bastards" wasn't given the same weight as the impending threat of the White Walkers, as Ser Davos explained to Lady Mormont that the real war will be against the dead, not Ramsay Bolton.
Basically, we need the White Walkers to get to Westeros eventually — and they've already run out of things to do Beyond the Wall following the Massacre at Hardhome.
Looking ahead, Sunday's season finale is aptly named "The Winds of Winter." Perhaps, in lieu of the episode title, the final scene of the season will feature the sweeping force of the White Walkers as they finally march south of the Wall following its destruction. As the Starks have so often reminded us: "Winter is [finally] coming."