Because conceptualisations of things as SKOS concepts are distinct from the things themselves. If this weren't the case, we couldn't have diverse treatment of common people/places/artifacts in multiple SKOS thesauri.I let you enjoy the rest of the post, and will simply add that indeed, it's addressing in the specific context of interoperability of SKOS and FOAF the very issue we've been speaking about here for years. Things are distinct from their conceptualisations, and we need a way for various representations to focus on the same thing they are about. There is still a step further, though. The thing itself is present in the information system only through representations. The URI for the thing itself is the best proxy we can get for it in the system, but let's assume with Dan and FOAF'ers that it's somehow closer to the thing itself than concepts in thesauri, and therefore allows the latter to focus on the former. And certainly I'm delighted with the metaphore of the focus, which is yet another avatar of convergence, like the spokes of the wheel I try to keep rolling here. In french, either spokes of a wheel or converging rays of light are called rayons.
I pretty much like the new property Dan Brickley introduced as the next addition to FOAF. For what it does, see Dan's explanations yesterday on LOD forum. The rationale is clearly set: