from Contract-First Web Services: 6 Reasons to Start with WSDL and Schema
I believe that there is a strong tendency for developers to think of Web services as methods. In reality it is not that simple. A Web service defines an XML conversation between client and server. Yet, when a developer sits down to create Web services he or she usually starts by creating a method.
From that starting point it is easy to get a software toolkit to turn your method into a Web service. By default or with a few settings you can allow the toolkit to define the conversation around your method. This method-first development technique works, especially for small or pilot projects, but it is not the best approach. A contract-first approach results in better long-term development, interoperability, and maintenance.