A basic item at a basic price:
What is a C47? Well, it's nothing more than a clothspin, not even some enhanced variation, just a regular old cloths pin. But what's it used for on a film set?
The grip/lighting department gets the most use out of them, the most basic function they have is attaching gels/diffusion/etc... to the barndoors on lights. And the only thing you need to keep in mind about them, is that you should never buy plastic ones, because plastic will melt on hot lights.
They're also cheap as chips, so you buy a bag of them and you're probably set for life unless you have a habit of losing them. They do come in handy for anything that might need clipping. Hey, maybe you have a sit down interview and cant use tape on outfit of the person you're interviewing, so you need some other way to secure the cable behind their back. Granted, i've never even done that, but i'm just painting a picture as to the fact that these things are so basic that their function can be used however you please.
The non productive function they also have are for games. The game that's been around forever is "how many c47s can i attach to someone's back before they notice" fun and games on a set are great for lightening the mood and keeping things cheery. Just be sure it doesn't impact your productivity, or piss someone off, or otherwise become destructive.