A .iwg file is an index file that the CAD Reader extension produces. This file is produced the first time you open a CAD drawing in ArcView GIS. ArcView GIS saves the index file in the subdirectory where the CAD drawing is located. Once ArcView GIS creates the .iwg file, the CAD drawing should take less time to draw. If you happen to delete the .iwg file, another one will be created the next time you open the CAD drawing in ArcView GIS.
The .iwg index file is only produced when working with .dwg files and not with other CAD formats.