Helps to prevent hot air from recirculating into the CPU cooler fan. The internal chamber of my case get very hot, even with a 120mm exhaust fan installed. On my Terra, it reduces CPU temperatures by upwards of 13C. Before installing this duct, my CPU was hitting the thermal limit (100C) running at only 125W. After attaching the duct onto my fan, at the same 125W setting, temperatures held stable at a max of 87C, and I am able to run the CPU at 140W for a max of 95C in the same test environment.

This particular duct is designed for a space of 17.5mm from the fan face to to the inside of the side panel. I measured using my caliper's tail end from the outside of the case vent slots to the fan face and then subtracted 2mm for the thickness of the panel.

Mine is printed in ABS, I would recommend printing in at least PETG, the case internal volume can easily get hot enough for PLA to warp and droop.