This thread really surprised me - I learned about arity when learning about ternary operators, which are pretty universal - you can find ?: in all sorts of languages. I think I was learning about C when I first got a detailed explanation on the subject.

Even if every function is curried by default (and "currying" is yet another concept that may be alien to users of some languages), it is probably still useful to know with what number of arguments a function can evaluate to a value that is not just another function