What does "self-describing" in reference to Web Services really mean?

I've heard a lot of buzz through the years about "Self Describing" web services and I'm curious what it means. I've flipped through the W3C standard and it doesn't really help.

I understand you can say "Give me this argument and call this functionality and I'll return something like this", but how is that actually helpful? How can software know what is relevant to a client given some context?

Can anyone give real world examples of this concept, and explain how it's better than some other alternative? Or maybe how useful/useless it is?

It's really designed as a standard for describing in a cross-platform cross-language manner an interface that a developer can use to develop a SOAP based way to exchange information with a web service.

Another alternative would be providing a library that provides a local interface to a blackbox communcation scheme, which is fraught with compatability/security issues.

Or providing documentation, which may be difficult to find, have compatibility issues, be out of date, or incomplete.

In short, it's very useful.