Perhaps you work in a multi CAD platform office or work with contractors or suppliers that use another CAD system like MicroStation V8. AutoCAD 2008 adds the ability to import, export, and even reference as an underlay the V8 format DGN file content/geometry.
The underlay option is a nice way to not convert or modify the V8 DGN file but base your design in AutoCAD DWG on the referenced DGN geometry. I created a short 2 minute video during the alpha and beta of AutoCAD 2008 then codename Spago and am posting it for your viewing.
Link to DGN Underlay Video (Windows Media Video "WMV" format 2 minutes, 10MB)