Beer changes flavor as it ages. For hoppy or aromatic beers that's bad because the desired flavor or aroma fades over time. However for a big malty beer, bulk aging lets the flavors develop in a very good way that gets better the longer you leave it. Even after bottling (assuming you don't drink it fast) you'll notice a flavor change from month to month. Best I've ever done is a barley wine that lasted two years. I bulk aged that for 4 months before bottling. The beer tasted very good after month or two in the bottle, but tasted fantastic at one year and even better at two!
Bulk aging (which is leaving the beer in the primary or secondary fermenter for months) is almost never about dropping gravity. I would suspect...meaning I'm guessing...that @squeegeethree's gravity drop at 6 weeks, was due to the kviek yeast.
Lagering would be another reason for a lengthy fermentation because the yeast are doing their thing at a slower pace.