I think that's what's called "biobale". On my bakpak I removed the biobale since I intend to use the live rock and live sand as my primary filtration. I would rather have the bacterial colonization in the rock and sand rather than in the skimmer as I want the anaerobic bacteria pouplations adjacent to the aerobic ones as much as possible. I also like to use the empty biobale chamber for other things. But that is only my opinion.
The biobale is not necessary, however it does provide some bubble trapping capabilities. It's not a very efficient biological filter so I wouldnt worry too much about that. That said, the second chamber does make for a nice area to keep your heater and run carbon.
