When I see this post, the first thing to pop up is SOAP as well. Just that SOAP is not human-readable. Then I suddenly remembered that it wasn't SOAP itself that include the schema but the SOAP providers would generate WSDL alongside a SOAP endpoint.

SOAP is an abstraction that is fundamentally unhelpful (at least in my opinion). Without SOAP, to call a webservice, as a developer, I need to read the documentation for the webservice to understand what data needs to go into what places. With SOAP, as a developer, I need to read the documentation for the webservice to understand what data needs to go into what places; but the documentation is much harder to read, and complex types are usually much harder to use (there's a tendency to model things as lots of complex xml, which are often hard to construct, instead of just a specified transform to a string)