It can be used to mark geometry which is visible in SketchUp as invisible when rendering with IRender.
One use if if you want to make a lamp or light fixture which illuminates the scene but which is not invisible.
In this example, the floor lamp is invisible, but still illuminates the rear wall.
Note: Invisible objects do cast shadows. (which makes them useful for canisters around lights which need to block the light.)
For example, here the table is invisible but still casts a shadow.
Here is an example of using an alpha-transparent material on an invisible face to cast shadows on the scene.