Its not so much I prefer them, more they have their place for certain jobs, my gardiners brush has two sets of holes so I have both types of jet fitted but not used at the same time.
My fan jets are the gardiner ones, but see the other thread because it seems they are not the best, I need them on full flow to spray well.
Find them good for applying tfr etc. from a backpack, first cleans on dirty upvc windows and make light work of leaded compared to pencil jets. Because the fine spray covers a greater area quicker where as with pencils you need to more accurate.
On regular maintenance cleans I work with a lower flow and find that 1.4 mm pencils work better for me and I save a lot of water.