Why is QMutex used in this code?

Question

I was reading someone else's code and I encountered this piece of code that is part of a multi - threaded application .This code is part of the run() function in a class which is inherited from QThread.
I thought QMutex is used for protecting a variable from being changed by several threads simultaneously . But here stopped is a member of the class. So it will be created for each instance of the class.
Am I wrong about mutexes? Has the programmer written a wrong code :) ? Or here mutex has another application?

void aThread::run(){
   aMutex.lock();
   if (stopped)
   {
     stopped=false;
     aMutex.unlock();
     break;
   }
   aMutex.unlock();
}

Here is the declaration of the stopped :

class aThread : public QThread{
public :
   void run();
private:
    volatile bool stopped;
}