Since ArcGIS for Server 10.1, the deployment include the optional component of the ArcGIS Web Adapter. Although the Deployment scenarios section in the help documentation does a good job of describing when to use the web adapter, this cheat sheet can assist GIS administrators and IT professionals to decide if the ArcGIS Web Adapter is required for their GIS architecture when installing ArcGIS for Server 10.1 and above.
A PDF is also available as an attachment for your convenience.