Redditor mn_lax_man speculates this is because Arya will eventually kill a White Walker, but there's still the possibility that she'll use it to kill Littlefinger, which has been the rumor for a while.
The writer has previously insisted that Stoneheart will not feature as Game Of Thrones builds towards its final eighth season, but fans remained adamant that she was present during Arya's brief exchange.
At the end of season six, the Starks have finally begun putting back the pieces of their family's shattered remains; Jon and Sansa are reunited and attempting to take back the north.
During that awkward encounter between Bran and Littlefinger, a dagger is passed between the two, and eventually to Arya.
The dagger Littlefinger (Aidan Gillen) gave Bran in "The Spoils of War", which Bran in turn gave to Arya, might be the most important weapon in the history of the show, in terms on its impact on the events that have transpired since season one.
Secondly, Littlefinger has to realize Bran knowing all means the young Stark knows Littlefinger betrayed Ned and sparked the War of the Five Kings.
Though Clapton doesn't explicitly state it, this is a clear reference to the horrific sexual assaults Sansa endured from Ramsay Bolton on the fifth season. Many fans are wondering how he will find out - and it's possible it will be with the dagger. In the Song of Ice and Fire book series, the Brotherhood Without Banners (the crew the Hound is now rolling with in the show) came across Cat's corpse shortly after the events of the Red Wedding and brought her back to life. She seeks revenge on the Freys for the Red Wedding, a role Arya has basically filled in Season 7.
The episode opens and closes with meetings between characters who had not yet met. But before they head off to the wrong side of the Wall, Sandor Clegane could see Arya Stark in Winterfell. Payne was Joffrey's royal executioner who beheaded Ned Stark with his own greatsword, Ice. Arya doesn't forgive. If she learns what Baelish did, it's over for him.
It is at ~exactly~ that moment where Arya brings up her mother that Reddit user GrennsGal pointed out that a ghostly figure strolls casually in the background.
"If Randyll croaks and Dickon survives there's a chance he could abandon his father's pact with the Lannisters, paving the way to reconnect with his brother Sam - who happens to have the Tarly's Valyrian steel sword and coincidentally is best mates with a Stark".