It was a signifier that White Walkers would be coming into play again pretty soon when Sam handed Jon a bag full of dragonglass last week, and it turns out it was going to be needed here. Naturally, when the attack happens, its the one thing they know can defeat the Others that gets left behind, just like leaving your house without your wallet or phone, only with more life-threatening consequences. Jon goes back to retrieve it, where he comes face-to-face with a White Walker. Swatted aside easily, and with the dragonglass out of reach, it doesnt look good for the Lord Commander. Then, when the Walker goes to strike the fatal blow, he throws up his sword in desperation, and actually manages to block it. Its hard at that point to work out which of the two is more surprised by the turn of events, but Jon uses it to his advantage as he defeats the White Walker with Longclaw. It turns out Valyrian Steel has the same effect on them as dragonglass, who knew?!