One thing about facets is it probably depends a lot on what kind of data you have - you have retail data you know very well what your facets are - shoes, men's wear, women's wear, accessories etc. etc.

If you have a large number of historical documents that have been scanned in you probably think you know what your facets are but there are also likely to be one's that you miss, thus you want to do some sort of facet extraction to determine what you have and combine that with the facets you have defined.

Obviously not every facet that is found will be ones you want, for example when I was at Thompson Reuters we did a facet extraction on all Danish laws and cases and one of the facets was Knife Killing, which was not useful to know.