When working with 3rd party xml web services (UPS, stamps.com, Chase, etc) it's often valuable to record the exact xml request and response communication between the service and integration client (your app). This information may be needed for auditing or debugging unexpected exceptions.
Since web services are xml based, SQL Server's xml datatype is the ideal solution to write the exact xml request and response values to.
If you're implementing the services using HttpWebRequest and HttpWebResponse calls, you already have the xml in-hand, and you simply need to write that to the database. However, if you are using proxy objects ...