Почему используется именно такая конструкция кода?

public final class ApiFactory {
    private static MovieService sService;
 
    public static MovieService getMoviesService() {
        //I know that double checked locking is not a good pattern, but it's enough here
        MovieService service = sService;
        if (service == null) {
            synchronized (ApiFactory.class) {
                service = sService;
                if (service == null) {
                    service = sService = createService();
                }
            }
        }
        return service;
    }
 
}

 private volatile Helper helper = null;

synchronized(this) {
                localRef = helper;
                if (localRef == null) {
                    helper = localRef = new Helper();
                }
            }

    private static volatile MovieService sService;
 
    public static MovieService getMoviesService() {
        MovieService localRef = sService;
        if (localRef == null) {
            synchronized (ApiFactory.class) {
                if (sService == null) {
                    localRef = sService = createService();
                }
            }
        }
        return localRef;
    }