The overflows dump into that compartment for a few reasons. The first is that I have 3 pipes coming down from the tank that can potentially be feeding the sump water. I wanted to be able to filter the water coming through those pipes with 1 filter sock. Putting 3 pipes into 1 sock makes it kind of hard to change out the sock.
Also, I wanted the main siphon overflow and main durso overflow to be submerged to reduce noise and bubbles. Submerging them and having a sock wrapped around them would again make it kind of hard to change out the socks - especially since the return lines are vertical without bends which would make it hard to "lift" them out of the sock.
So basically I wanted a cleaner solution where the pipes weren't sticking into the sock but all the flow still went through the sock. I got the idea from an
Aqueon Proflex sump I saw online - but I wanted one to accomodate multiple pipes and 1 large 7" sock. It turns out RC member slief has done this as well and
his sump helped confirm my design.