To paraphrase both Ecclesiastes and Michael Stonebraker & Joseph Hellerstein, there is nothing new under the sun.
It's as true with Semantic Web technologies as with anything else—tuples are straightforward, ontologies build on schema languages and description logics that have been around for ages, URIs have been baked into the Web for twenty years, etc. But while the technologies are not new, the circumstances are. In particular, the W3C set of Semantic Web technologies are particularly valuable for having been brought together as a common, coherent, set of standards.
- Common. Semantic Web technologies are broadly applicable to many, many different use cases. People use them to publish pricing data online, to uncover market opportunities, to integrate data in the bowels of corporate IT, to open government data, to promote structured scientific discourse, to build open social networks, to reform supply chain inefficiencies, to search employee skill sets, and to accomplish about ten thousand other tasks. This makes a one-size-fits-all elevator pitch challenging, but it also means that there's a large audience of practitioners that are benefitting from these technologies and so are coming together to create standards, build tool sets, and implement solutions. These are not niche technologies with limited resources for ongoing development or at risk to be hijacked for a purpose at odds with your own.
- Coherent. Semantic Web technologies are designed to work together. The infamous layer cake diagram may have many shortcomings, but it does demonstrate that these technologies fit together like jigsaw puzzle pieces. This means that I can build an application using the RDF data model, and then incrementally bring new functionality online by adopting other Semantic Web technologies. Without a coherent set of technologies, I'd have to either roll my own solutions for new functionality (expensive, error-prone) or try to overcome impedance mismatches in connecting together multiple unrelated technologies (expensive, error-prone).
- Standard. Semantic Web technologies are developed in collaborative working groups under the auspices of the World Wide Web Consortium (W3C). The specifications are free (both as in beer and as in not constrained by intellectual property) and are backed by test suites and implementation reports that go a long way to encouraging interoperable tools.
The technologies are not novel and are not perfect. But they are common, coherent, and standard and that sets them apart from a lot of what's come before and a lot of other options that are currently out there.