bio balls?

Throw some live rock rubble in there where the bio balls would go. You can just use base rock and break it up. It will do much better.

 

The sump adds additional water volume, a place to keep your heaters and skimmer. It also allows you away to add an automatic top off system. A refugium can be added as part of your sump. You can add chaeto algae to your refugium which will gobble up the nitrates in your system and prevent algae from growing in our display tank. Since the chaeto got established in my refugium my nitrate have stayed below 1 ppm.

 

A sump is a great place to put large equipment which would get in the way and look unsightly in your DT. Things like your protein skimmer, heater, activated carbon and if you wanted bioballs could be placed in your sump. It also helps when topping up your system with freshwater or additives etc as they can be added directly to your sump and these additions can then mix with the water in the sump before reaching your DT so it doesn't shock your inhabitants in your DT.

A lot of people isolate a section of there sump to be used as a refugium with a light above it to cultivate seaweeds and to provide a safe place for pods.
Even if ou don't use it as a refuge, and keep your sump in the dark, you will get a lot of pods living in your sump as well as sponges which will help remove organics.