As for van Gogh's "importance to society," I always shudder when students ask about that. I think some teachers are misleading them, so that they are confused about why they are studying art history in the first place. The truth is that van Gogh was very important to the history of art, that is, to cultural history, but neither he nor any other artist was vastly "important to society" at large. "Society" would have wended its way along perfectly well if he had never existed, though the history of art and culture would have been the poorer for his absence.
I'm an art historian, so you can take my word for it when I say that.
