Raytracing is a global illumination algorithm, where the interaction of photons between neighboring objects plays an important role in enriching the realism of the scene. Therefore, adding context to the current scene can greatly improve and enhance the realism. Auxiliary geometry (being able to add floor, walls, and or ceiling on which textures can be mapped, or shadows or reflections can be cast) serves this purpose. Below is a comparison, where a floor is created for the right image, while the left has no floor. Shadow and reflection make the shuttle image on the right more realistic.

For a quick guide on how to use the auxiliary geometries, see Create Auxiliary Geometry.