So far I've only had experience of rendering from Maya on a single render layer.
It's beneficial to use different layers as it allows a greater degree of control over the final look of the product.
To practice this, we aimed to place additional assets (some alphabet blocks) in the bottom photograph as seamlessly as possible.
Firstly we need a Colour Pass to capture the textures of the blocks. We created a new render layer and added the selected relevant geometry.
The blocks on the right are on a reflective surface, so we need a Reflection Pass on another render layer. By making the Blinn shader on the board as reflective as possible, we have maximum control over the level of reflectivity.
We only want the blocks to be casting shadows, though we need to add the table to the layer as that receives them. The shadow pass is an alpha layer, so we need to change the preset of this new render layer. To do this, we right click on it, then hit 'attributes' and click 'shadow' underneath the 'presets' menu.
When you render a frame, you can click to display the alpha channel to see the shadows.
Finally we create an occlusion layer, which creates a realistic 'self shadowing' effect. We add the blocks to a new layer, and this time under the 'presets' menu we click 'occlusion'. Under the render settings, we need to right click Image Format and create a new layer override, in this case a JPEG, to take away the transparency so the image can be used as an overlay without creating any abrupt edges.