It has importance, actually, in a way that I don't fully understand. If memory serves, Robert's kid Marley says "Daddy look it's a butterfly!" while boarding the chopper. So, then, the infected dude happened to smash the glass into a shape similar to a butterfly, then, Maria I think is her name, has a tattoo of it. I think this is all relating somehow to Maria's "God's Plan" speech, which is why Robert says "I'm listening." before locking Maria and Ian into the..uh, fireplace. I think it makes sense, however, like the rest of this movie, it requires you to think very deeply. I Am Legend expert I may be, but I may have to work on this one.