Recursive functions are indispensable for the processing of recursive data structures
The problem solving technique sketched here is called divide and conquer. It is not all divide and conquer problems that involve recursion. But many do in fact. Recursion comes into play when the subproblems P1 ... Pn turn out to be of the same nature as the overall problem, and as such they can be solved by the same 'medicine' as used for the overall problem P.