I’d argue that like a minor league baseball team, a minor character has the capacity to transcend their supporting role. Maybe even rebel against it. If the writer’s not careful—maybe the writer should sometimes take care not to be so careful—a minor character might become interesting in and of themselves and, in this way, offer the reader a necessary break now and then from the spotlight-hungry lead.
Even as a kid, I remember reading stories and being enamored not only by the characters, plots, and settings but also by the notion that there was a personal presence behind the telling, that someone out there, some author, had first imagined into being what I as the reader was now imagining into being. As much as I loved reading, this idea of being the first imaginer was something that I always aspired to, and I was lucky as a young writer to have had many good teachers—from elementary school all the way up through my graduate programs—who cultivated this aspiration.