It's been previously established that Tanjiro has a keen sense of smell and seen to always comment on the pungent smell of demons or the emotional states of people. In his very first training arc, he was mentored by Urokodaki, a former Hashira who also has a keen sense of smell. Then suddenly Tanjiro has a supernatural experience while training with the dead students of his master. So does he master his sense of smell to use in combat? Nope! He gains the ability to sense the "opening thread" i.e. The ability to sense when the opponent is left open for a finishing blow. But what does that have to do with any of the previously established plot points?
What was the point of the "opening thread" in the story? Did it have some sort of significance?