Well, the advantage is that applications that wish to integrate with such data need not make the declaration as part of the application. This particularly applies to things that don't understand taxonomy, things that browse the semantic web in general. It allows things like that to look at a whole world of URIs that identify tc taxa, a whole world of uris that identify DwC taxa, and go "hang on, these have something in common".
Another purpose of these kinds of declaration is to answer questions like: "Ok, what *kinds of thing* might be meaningfully said about the scientific name Fabaceae"?", or "What existing vocabulary terms describe teeth?"
This might be particularly applicable when people start minting their own vocabulary terms.
If the herpetologists get together and produce a vocabulary for describing reptile scales, by linking their vocabulary to existing well-known generic ones (eg, the "old" tdwg terms) it becomes possible to build editors that are "smart", that understand that if you are defining a DwC specimen record, then "scale shape" is a predicate that you might want to apply to it.
The herpetology vocabulary can contain specific things that would not appear in TDWG: for instance, the class "scaled thing". The moment you declare that a thing has a herp:scaleShape, then we infer that it is a herp:ScaledThing, and that therefore it might/must also have values for certain other properties. In addition, an application can group the properties relating to scaledThings together, rather than having all possible properties as a alphabetically sorted list. That is: the vocabulary does something useful beyond being simply a list of field names.
Perhaps it might be put like this: it's these declarations that put the semantics in the semantic web. Without them, you simply have linked data.
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