C++ :: Multithreading - Demonstrate Use Of Semaphores

I have come across below code to demenostrate the use of semaphores

#include <pthread.h>
#include <semaphore.h>
#include <stdio.h>
#include <stdlib.h>
#define NITER 1000000
int count = 0;
void * ThreadAdd(void * a)

[Code] ....

Why the question of syncronising threads and mutexes and semaphores come into the picture when we join threads?

Like in the above example we have joined the first thread and then second thread so that main process will pause for the first thread to finish as its joined and then main process resumes and gives control to second thread and pauses till its finished as this thread is also joined.

Then where is question of simultaneous access?

First thread does its job and ends itself.
Second thread does its job and ends itself.

I wonder why there is need to use semaphores or mutexes in this example? I am very new to Multithreading and this question bothers me as to why we have to worry about simeltaneous access when we can join threads and can make control wait till that thread ends?