Many learners make errors during initial learning of composite functions.  Substituting the wrong way around etc.  This task aims to help draw attention to the core idea.  While normal procedurally focused exercises are important, this sort of task is complementary to those and can help to move the learning forward. 

A nice extension would be to have pupils come up with a pair of functions of their own and then make up some cards for the various composition permutations of them.  

Credit: @chrismcgrane84