Perhaps you’re looking into web services and you’re having trouble understanding what’s the point of a WSDL document? I did a little research on this, and it seems to me that the main use of WSDL is to provide the web service interface so that software development tools like Visual Studio can generate code for working with that interface. Also, it seems like this is very common with SOAP-style web services, but not REST web services (because REST is so simple).
Here are some references.
A main idea behind wsdl is that the wsdl definition can be used to automatically generate service side skeletons that can interact with the deployed object that provides the service and to generate client side stubs that can communicate with the skeletons, thereby providing the client program with access to the functions provided by the service. Make the wsdl for a service available and you open the door to fast, easy programmable access to that service.
“Big Web services” use Extensible Markup Language (XML) messages that follow the SOAP standard and have been popular with the traditional enterprises. In such systems, there is often a machine-readable description of the operations offered by the service written in the Web Services Description Language (WSDL). The latter is not a requirement of a SOAP endpoint, but it is a prerequisite for automated client-side code generation in many Java and .NET SOAP frameworks (frameworks such as Apache Axis2, Apache CXF, and Spring being notable exceptions).
If you’re studying web services, I highly recommend RESTful Web Services, by Leonard Richardson and Sam Ruby.