Floss is used for mechanical filtration, to help clean up the water visually especially if you are doing a lot of digging around and kicking lots of gunk up into the water column. As for biofiltration, it wouldn't be any better than a sponge and would probably be worse.
Only time I have ever used floss in any of my HOB filters has been at initial tank setup when the water was really, really cloudy (due to using flourite as my substrate). Never had a need to use floss after that.
