it's all about the Performance Testing
SOAP was originally part of the specification that included the Web Services Description Language (WSDL) and Universal Description, Discovery, and Integration (UDDI). It is used now without WSDL and UDDI. Instead of the discovery process described in the History of the Web Services Specification section below, SOAP messages are hard-coded or genereated without the use of a repository. The interaction is illustrated in the figure below.
The SOAP envelope contains two parts:
SOAP commonly uses HTTP, but other protocols such as Simple Mail Transfer Protocol (SMTP) may by used. SOAP can be used to exchange complete documents or to call a remote procedure.
NOTE: SOAP at one time stood for Simple Object Access Protocol. Starting with SOAP Version 1.2, the letters in the acronym have no particular meaning.
More information: SOAP page on the W3C website