Game Of Thrones Season 7 Reactions: What "Dragonstone" Really Means
3. Dragonstone's Precious Stock
It might not have been that much of a revelation to us as an audience, but the episode confirmed that there's an almighty stock of Dragonglass in Dragonstone, which is no doubt what draws Jon Snow to Dany's shores. Or which compels him to be more receptive to her demands for fealty (should it come) than he was to Cersei's.
In rejecting Cersei's call, Jon professed that he had bigger fish to fry and that the distance between King's Landing and Winterfell (and not to mention the climactic differences) would protect them anyway. The same should be said of Dany, but the fact that she has Dragonglass makes her a far more useful ally to Jon than Cersei could ever be.
As soon as he gets that raven from Sam, he's going to head to Dragonstone to get his hands on some of that valuable mineral.
Incidentally, it's odd that Sam got so excited as if this was fresh information, given that Stannis literally told him the Dragonglass was on Dragonstone ages ago...