What is the use of setDesignResolutionSize in cocos studio?

In my appDelegate class i have code something like this, while using cocos studio.

auto glview = director->getOpenGLView();
    if(!glview) {
        glview = GLViewImpl::createWithRect("HelloCpp", Rect(0, 0, 960, 640));
        director->setOpenGLView(glview);
    }
    director->getOpenGLView()->setDesignResolutionSize(960, 640, ResolutionPolicy::SHOW_ALL);

Where as in a normal cocos2dx project with out cocos studio, the code in appDelegate class is like this.

auto glview = director->getOpenGLView();
    if(!glview) {
        glview = GLViewImpl::create("My Game");
        director->setOpenGLView(glview);
    }

My doubt is, is it mandatory to setDesignResolutionSize, and also should it be the same for every device size???