A plot or subplot follows the efforts of the protagonist within that plot to achieve his primary goal. He does this by attempting to accomplish all the lesser goals necessary to achieve the primary goal. (If he is going to kill someone, he has to go get a gun first.) This path to the primary goal is the plot's throughline. It is vital to the life of your story.
Recognition of this throughline by the reader gives the reader confidence that there is profluence to the story. Profluence is the clear sense that things are moving toward something, getting somewhere, flowing forward, have a direction and a destination. This recognition early in the story is vital to keeping your reader reading.
A good test of causality is to ask yourself: Could I move this scene to someplace earlier or later in the story? If the answer is yes, it just might not be causally linked.