I know Stephen Wolfram is a bit of a contentious figure, but his write-ups are always absolutely incredible. And if you're wondering why combinators are important (particularly S and K), SKI is a universal formal system[1]. Which is kind of crazy to think about: just two axioms and modus ponens can compute anything (I is just SKK).
[1] http://people.cs.uchicago.edu/~odonnell/Teacher/Lectures/For...