At first they were strictly worn for formal wear but lately I have branched out to wearing them just to wear them, my black bows would just be for formal wear, something like this:
I've also taken to wearing a bow tie at least once or twice a week. I always get comments on it I guess because so few other men are wearing them and they're a bit of a novelty. I live in a college town and I see a few guys wearing them on campus though. By the way, great pocket square!