The Scene Layout Window is a conventient place to arrange any 3D models you're rendering and place any lights you might have. While you can move models around the scene using the render pipeline state and render pass views on the Shader Path window, you may have cases where not all your models are part of any one render pass or pipeline state. Having all the models in one place makes it easier to see how they're laid out in the scene and move them as needed.
Note that a 3D model isn't visible on the Scene Layout Window until it is added as a shader input to at least one render pipeline state in your project.