The sketchbook is a powerful tool allowing you to quickly sketch thoughts and ideas outside of the core modelling environment.
However, an even more powerful aspect is the ability to use it as traditional trace/butter paper.
Quickly mass up your 3d model, then send a copy to your sketch book for you to trace over.
You can repeat this process as often as you like while your project advances.
Tracing over the model allows you to create sketch based presetation drawings, or to explore your ideas before reverting back to modelling.
To send a 3d view to the sketchbook simply set up the required view point and use the Send 3d to Sketch item in the Import/Export Menu.
The image is placed on its own layer.
You can trace over the top to enhance the underlying drawing, or trace completely and then hide the background layer to provide an accurate sketch based on the base model.