HDRI maps used in SketchUp and rendered with Vray allow you to set up lighting and reflections. The problem with rendering reflective objects in an otherwise empty scene is that there is nothing to "reflect" off the object. With HDRI maps, you can create a scene without actually adding additional geometry.
For a very helpful, quick and simple tutorial, see SketchUp Artists' "Using HDRI in V-Ray for SketchUp." Effects, blurring and some shadows were added in with GIMP after rendering was complete.
