When to use static instance of class in c++ ?

Qt embedded developer

I am reading one project code there i have seen that in several places they use the

like in thread related class , database access related class.

I am Not knowing the why they use the static instance of class.


#ifndef SRC_OTATHREAD_H_
#define SRC_OTATHREAD_H_

#include <queue>
#include <vector>
#include <pthread.h>
#include <evt/libevt.h>
#include <boomer.h>

namespace app
{

namespace otathread {

enum OtaThreadState {START=0, STOP=1, CONTINUE=3} ;

class OtaThread
{
public:
    static OtaThread *getInstance() ;

    void initImpl();

    int initAndStartThread() ;
    void stopThread();
    void checkForOTAThread();
    OtaThreadState runImpl(OtaThreadState) ;
    pthread_t _thread;
    int getBackSyncTimeout();
    static boomer::transaction::TransactionResult SyncTimeOutCallback(void *NotUsed, int argc, char**argv, char** azColName);
private:
    OtaThread();
    ~OtaThread();
    static void *runThreadFunc(void *This) ;
    void run(void) ;

    Event mEvent;
    int OtaTimer,defaultTimer;
    int deviceTimer;
    static OtaThread *otaThreadInstance;
    static bool otaThreadActive;
    static int NumberofOTAcall;

};

} /*namespace otathread*/

} /*namespace app*/
#endif /* SRC_OTATHREAD_H_ */