Libgdx，何时使用camera.position.set？

I am really confused with two examples related to viewport and orthagraphic. Although i understand that Viewport is the size of the dimensions we set to view on the screen and camera projects that. I am learning libgdx and cannot finish through orthographic camera and viewport examples which have left me completely confused. the code runs fine for both examples and with proper result on screen.I am really confused with two examples related