One of Nutch's key selling points is its transparency. Its
ranking algorithms are open source, so anyone can see them. Nutch's
ability to "explain" its rankings online--via the explain
link--takes this one step further and allows an (expert) user to
see why one particular hit ranked above another for a given search.
In practice, this page is only really useful for diagnostic
purposes for people running a Nutch search engine, so there is no
need to expose it publicly, except perhaps for PR reasons.