Dairies use them to transport milk from the bottler to your local grocery store.

College students use them to store books. I've always heard that it was illegal to use them for anything other than transporting milk; if this is really so, it's the dumbest thing I've ever heard of.

rougevert points out what should've been obvious to me: they are, in his words, "the universal portable chair, popular with the homeless and the young especially." Because who, when you really get down to it, doesn't enjoy a good sit?