Why use RDF instead of just XML?

Nice comments from Dan Brickley in response to this posting HubLog: RDF braindump.
This question keeps resurfacing, and this is one of the best answers I’ve seen:
“One way to think about this: the Resource Description Framework (RDF) is a family of XML applications who agree to make a certain tradeoff for the sake of cross-compatibility.”
Via RDF IG Scratchpad.