Why does start_routine for pthread_create return void* and take void*

The function header for pthread_create looks like this:

int pthread_create(pthread_t * thread, 
                   const pthread_attr_t * attr,
                   void * (*start_routine)(void *), 
                   void *arg);

I understand it all except that the function pointer for start_routine is of the form void* (*fpointer) (void*) which means it takes in a void* and returns a void*.

The void* parameter is just a way to pass in an argument to the start_routine, I get that part, but I don't understand why the function returns a void*? What code will even notice that return value?