It's common to mock superstitions to display one's Skeptic™ cred, but it's a mistake, because they are Chesterton fences.
It's bad luck to walk under a ladder. No, okay, it's not "bad luck", but ladders are dangerous.
It's bad luck to open an umbrella indoors. Or more accurately, it's inconsiderate, because open umbrellas are deceptively difficult to handle indoors and you're pretty likely to bump into someone or knock over a lamp. A fifteen-minute training course on umbrella safety and etiquette wouldn't be a waste of time.
Seven years of bad luck for breaking a mirror. Not really sure about this one but who wants broken mirrors? Mirrors are probably pretty hard to make.
I've heard that in some Islamic countries it is traditional to touch food only with the right hand, and reserve the left hand for disgusting activities like wiping the ass. For years I thought this was some sort of unprincipled discrimination, but somehow I spontaneously saw the purpose one day. It makes perfect sense to use only one hand for eating in a world where you can't wash your hands right before every meal. The practice makes enough sense to me that I do it.
Cultures exist in a world of slow meticulous selection pressures. Like organisms, they must adapt to them to survive. This is the basic tenet of the study of cultural evolution. It's a small insight, to realize that superstitions are part of culture, and thus also subject to, and created by, those pressures.
My hope with this article is that it produces a Baader-Meinhof effect in you for a little while, so that when next you see someone call some habit a 'superstition' you can think about whether or not there's a pretty obvious purpose like in the above four examples. But there may be a purpose even if you can't think of one, for reasons articulated here and in much more depth and generality here.