"The experimental test of whether this art is great or good or minor or abysmal, is the effect it has on your own sense of the world and of yourself. Great art changes you." - Sister Wendy Beckett

 This could be called a cause and effect situation, a piece needs a cause, a purpose to create an effect on someone. Art commenting on social or political issues will often change how people see these issues & hopefully compel them to act. Even if a piece is just made to express a feeling, it can inspire people, change how they think or connect to it.