32 bit Z-buffer is in very many cases insuficient in outdoor virtual environments.
I often find that if I want the avatar to be human like, and move as close to
walls as a real human is able to, I have to have the front clipping distance
very close in order not to clip the wall in front of me. In outdor environments
it is often desirable to be able to view several kilometers and therefore the
frontclip and the backclip distance is very far.
The problem with this is that the spacing between layered polygons have to be
very big in order to avoid z-fighting. This in itself is irritating, but on top
of that this gap could become so large to be viewable from an angle.
Layering polygons is a desired modeling technique since it can save reduce
polygon counts considerable, and also speeds up modeling in many cases.