Use of MPI_COMM_SELF

I've discovered an MPI communicator called MPI_COMM_SELF. The problem is, I don't know, when is it useful. It appears to me, that just every process "thinks" about itself as root.

Could you explain me how does MPI_COMM_SELF exactly work and in which situations is it useful?

I've found this slide-show, but the communicator is only briefly mentioned there.

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I've tried this "Hello, world" example and all processes returned 0 as their PID.

#include <mpi.h>
#include <stdio.h>

int main() {
    MPI_Init(NULL, NULL);

    int world_rank;
    MPI_Comm_rank(MPI_COMM_SELF, &world_rank);

    printf("Hello, my PID is %d!\n",
            world_rank);

    MPI_Finalize();
    return 0;
}