A lot of people use them as bio media, as they hold a lot of bacteria. I've never used them myself, I prefer SeaChem's Pond Matrix. Large size, doesn't float, and also holds tons of bacteria.
more surface area then bio balls. every tank i own has scrubbies as bio media. none use real fish store bought bio balls or any kinda real bio. just scrubbies n walmart filter padding.
how can you be sure it works? bio builds up on any surface with flowing water. you could put brokeup pieces of plastic, or a buncha lava rocks, or cut up straws. nylon scrubbies work great man trust me. i have over 50gals of scrubbies as bio on my 400gal and my tank couldn't be more clear.
I've got an old whisper power filter laying around that I got tired of buying cartridges for...might be cool to stuff a few of those in it instead for extra bio filtration?