Sumps/refuges are always good to have,more water volume, place to let pods grow safely,place to put a mean fish while you find it a new home or allow a newcomer to adjust to the main then reintroduce the meanie,grow macroalgae/nuisance algae to reduce waste,add a DSB for nitrate reduction,hide equipment,between overflow and the actual sump there is alot more surface area for gas exchange added, and the better protein skimmers are in-sump.
The only downs are setup,possible leaks in the plumbing...and....IDK lol:wink1: