A gray-scale rendering, which is also know as a cardboard rendering, is simply a 3D model without textures and materials. The reason for this is, once I start to add colors and materials, change becomes more difficult. The gray-scale rendering isn’t worrying about lighting or composition, it is to make sure the model is accurate.
I have had projects that started off with the wrong drawing files. With a gray-scale proof, this kind of mistakes can be caught early.
The gray-scale rendering also doesn’t show the site or environment. The reason for this is, if the building footprint changes, the site and environment, which is often the most complicated part, doesn’t have to be rebuilt.