Why Are There Functions?

Returning from giving my Polymorphism Unbound presentation at StrangeLoop (not yet available on YouTube, but all the examples and presentation slides are on Github), I found myself dissatisfied. Part of this was certainly my failure to cram a 2-hour presentation into 40 minutes (after cutting it down from a day-long, workshop-length size). The bigger issue was my inability to answer the essential question at the end of the presentation: why are we writing polymorphic functions?