One use of the <unit> tag is to allow cataloguers to display metadata with some relative distance to the displayed record. We could call this the Six Degrees of Kevin Bacon for CollectiveAccess.
Let's say you're creating a Collection summary and you'd like to display metadata about Entities, but Entities aren't related directly to Collections. Perhaps they are related to Objects related to the Collections. Never fear! Just as Bruce Willis is 2 degrees away from Kevin Bacon (thanks to Brad Pitt, "The Hamster Factor" and the movie "8"), Entities are also 2 degrees away and their relativity to Objects relative to Collections can be exploited.