It's joining the two graphs so that you can do a query like this:
SELECT ?parentName WHERE
?child name 'Brendan'
?parent parentOf ?child
?parent name ?parentName
You're being quite patient with me, thanks. :)
SELECT ?son WHERE
<brian> parentOf ?son
?son gender <male>
The difference between querying and inferencing isn't what I was trying to emphasise, though. My point was the difference between being designed for making queries/inferences within a dataset, and being designed for joining distinct datasets.
Querying within a dataset is easy: SQL, XPath, XQuery, LINQ, etc. You can write rules for transforming any data model that you can query.
RDF isn't anything special in these areas (though I do think that SPARQL is an awfully nice query language). What it gives you is a way to link and merge datasets.