But more importantly, the B-plot forces a test on them. They've grown through the course of the plot, and now have their priorities straight. Not so much, perhaps, in the initial events. But if the A-plot is the finding of the treasure, then the romance doesn't necessarily conclude happily. In fact, Jessica could turn out to be his adversary, a spy in league with the villain, so that any love he begins to feel for her is a conflict in the A-plot because it diminishes his desire for the treasure and weakens his resolve.
Love is a danger and a cheat.
You might see the B-plot as the workhorse plot, the one that does most of the pulling and pushing of conflict and event. It is essential, in that the story would probably be two-dimensional without it. Barb, a subplot is a storyline that starts, develops, and resolves within the other plots. It usually starts sometime after the opening of the two major plots, and resolves before the climax. It supports one or both of the major plots, perhaps by providing additional conflict causing events teaching the protagonist some skill or lesson forcing an internal conflict to the surface Looking at my example above, of John and Jessica, I see a subplot in Jessica's relationship with her tyrannical Uncle Parham.