How is SOAP Used?
There are many possible applications for SOAP, here are just a couple:

• Business to Business integration – SOAP allows businesses to develop their applications, and then make those applications available to other companies
• Distributed applications – programs like databases could be stored on one server and accessed and managed by clients across the Internet

One thing to consider when looking into implementing SOAP on your business server is that there are many other ways to do the same thing that SOAP does. But the number one benefit you’ll gain from using SOAP is it’s simplicity. SOAP is just XML and HTTP combined to send and receive messages over the Internet. It is not constrained by the application language (Java, C#, Perl) or the platform (Windows, UNIX, Mac), and this makes it much more versatile than other solutions.

source: webdesign.about.com/library/weekly/aa031802a.htm