On occasion there may be a desire to give a post a category (hence it's own RSS feed), but don't really want that category visible.
For example if a blog's post from category 'foo' are syndicated to planet.foo.tld. Perhaps a post about foo shouldn't be syndicated but still categorized as 'foo'. Well if planet.foo.tld points to the hidden category, one can still categorize as 'foo' and not check 'fooplanet'.
This would enhance the ability to offer feeds for various purposes without hindering/cluttering in some cases. Nobody wants to see several foo categories.