Its really interesting to know even vast great scenes with beautifully designed environments and buildings were not based around the art but the art round them. I remember seeing behind the scenes on games where they had the grid style walls and colourless objects, most memorably in Ratchet and Clank. In the game there was a whole secret 'museum' of behind the scenes artefacts and play test items to play with. However even after seeing these I imagined these were just untextured and simplified for testing and that they were just toggling the art on/off as they worked.
I suppose this is so they can know if puzzles/jumps can be completed etc.
this makes an artists work even more interesting with a new level of problem solving they have to achieve. I love stuff like this and which I knew about it earlier. I cant wait to move on to UDK projects and really get to feel like I'm part of a bigger picture.