Usually, when you think of a Web Service, you think …make the call, get the response, and get on with the task at hand. These "one shot" calls are the norm in Web Services but there may be times when you need a little more. You may need the Web Service to remember states between calls.
On the ASP.NET forums where I moderate, the poster's IP address is displayed at the bottom of the post. Often I am curious and paste the poster's IP address into an IP locator web page to find out where they are posting from. This was tedious.