The first is querying. By querying a database, rdf store, etc. through a query you can only transmit the triples that meet your criteria.

The other is being able to access RDF models externally. You could query across different data sources such as RDF databases, SQL databases, RDF files on your local hard, RSS feeds off the web, etc.

Some of these data sources can support having the amount of data returned reduced by a query, the others need to be parsed, stored and then queried.

Hopefully it's the small, quickly changing, RDF sources, that don't support querying and not large , quickly changing RDF sources.