Site relevancy vs. page relevancy
Since the emergence of Google as the dominant player in the search engine marketplace, SEO practitioners have concentrated primarily on "page relevancy". The reason being that Google utilises a system called "Page Rank" as one of the factors in determining the order of its SERPs (see later section Google Page Rank).
Thus the "on-page" factors that people manipulate to achieve a good ranking are generally page-specific, with each page being treated as an entity in its own right - and thus individual URLs serving as the destination pages for click throughs from the SERPs.
This is still the case for the most part, except that Google last year indicated that the "quality" of an overall site was also important for its ranking methodology.
In practice, what this means is that a site that is about general photography-related issues should rank higher for the search phrase
photography
than a site that is about marketing, but which has a page featuring info relevant for photographers.
Our research and analysis has determined that a subject-specific section of a site that is generally considered to be "credible" and "authoritative" overall - see explanation in the later section Credibility & Authority - will have this specific section treated as though the whole site is about small business issues.
Thus the V&A site - being generally perceived to be "credible" and "authoritative" - passes on that "credibility" and "authority" to its Photography section.
This enables a site such as the V&A's to be listed very highly in the SERPs for multiple search phrases, without having to set up multiple unique web sites dedicated to specific subjects. I.e. the "site relevancy" factor is already in place by dint of the site as a whole being deemed "credible" and "authoritative".