The combination of the external conflict, the main relationship and the internal conflict is the basis for the protagonist’s journey; together, those aspects of the story constitute the main plot. But both the main plot and the protagonist’s journey can be enriched with subplots. These are minor yet complementary storylines centering around minor conflicts with supporting characters that arise as the main plot plays out.
In dealing with these minor conflicts, the protagonist will learn multiple lessons that in the end allow them to overcome their misbelief, ultimately giving them the courage to risk everything to achieve their goal. This makes the story’s subplots integral to the protagonist’s journey — removing a subplot should have a profoundly negative impact on the overall story. If you can remove a suplot without such an impact, then it should most definitely be cut. It is a story’s subplots, then, that allow you to fully and richly explore the story’s themes and create a truly character-forming and life-changing journey for the protagonist.