Why are function pointers useful?

main.c

#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#include <time.h>

int sum(int x, int y) {
    return x + y;
}

int prod(int x, int y) {
    return x * y;
}

void shouldNotBeChanged(int (*operation)(int, int)) {
    srand(time(NULL));
    int a = rand() % 100;
    int b = rand() % 100;
    printf("The result of the operation between %d and %d is %d\n",
        a, b, operation(a, b));
}

int main(int argc, char* argv[]) {
    shouldNotBeChanged(&prod);
    return 0;
}