Passing functions as arguments is useful when you have something you want to do $($a function$),$ but you want some other complex piece of software to control when that function is called. MapReduce is a technique for processing large data sets in the cloud; you provide the functions and the MapReduce system spreads them over multiple machines. Another example would be a graphical application. You may want a function to be called when you press a button. You let someone with knowledge of GUI code implement the button, and you pass your function as an argument.

Can you think of any other applications of passing functions as parameters?