RDoc XML templates: yes, but what's it for? {scratch head}

So I'm trying to provide a bit more documentation for RDoc, and I'm 
starting out nice and small, with the intent of building up to bigger, 
more detailed information. But I've already got a puzzle that hopefully 
somebody can help me out with.

As you may well already know, RDoc can output in a variety of formats. 
Specifically, "ri" format, HTML, CHM (which appears to be a 
Windows-specific Help system format), and XML.

XML has two templates available, the default (xml.rb) and rdf.rb.

I've added three lines of documentation to rdf.rb. (oooo!)

# This template is invoked by calling rdoc with the "--fmt xml"
# and "--template rdf" options, and results in an XML document
# in the {Resource Description Framework}[http://www.w3.org/RDF/].
#

OK, I guess it's four, including the blank. I've read a bit of the 
w3.org site, but I still don't know what it's *for.* Can anybody tell 
who are the people who say to themselves "Oh, yea, that's exactly what 
I need! My Ruby Documentation in RDF!" and why they say that?

Likewise, is the default XML template format used for some specific 
purpose?