All needle files are for finishing to one degree or another. I only buy #2 needle files (although I did buy a #4 pillar file once), but finer ones would be for finer finishing that you might normally do with abrasives. Think of a #4 or a #6 Swiss cut as more of a scraper or burnisher. I have small stones for finishing, but I guess that a #6 needle file would probably replace my 320 grit stone; just a guess. You wouldn't have to worry about wearing out a needle file like a stone.
You know, I imagine a #6 file might make more sense on silver or gold for jewelry work.
About cleaning it, I use a piece of sheet brass on end to clean my fine files. You rub the brass sheet along the teeth, the piece of brass is perpendicular to them, such that the teeth cut grooves in the brass. That cleans out the areas between the teeth. That method should work for any fine toothed file, even a #6.