You can make changes to the scene, re-render the passes, and immediately view the updated re-combination through the Render Target.
In this example, the camera direction is changed and the passes are re-rendered. When you switch to the Render Target in your Render View, a re-combination of the updated render passes appear.
Now, when you render the render target, all passes and layers that are upstream of the render target (in other words, those feeding images to the render target) are re-rendered, and the compositing graph then updated.
The Render View shows a re-combination of the updated render passes.