IF your talking about those specialized external filters I've found that they are a pain in the @$$ to maintain, but effective, when they work. That said, you can probably get just as good results with a skimmer and a reactor full of carbon.
and I already have it and I used to use it way back when I started with saltwater but I don't remember how it helped because I was just a mess back then. I was wondering if I should put it back on if it'll benefit
That's the one I had experience with. If I remember correctly it's like having a super live sand bed; since the sand is kept in motion/suspended it provides a lot more surface area for bacteria/water contact and hence better biological filtration. That's what I remember, I may be wrong.