This is a reducer for using with the Wall & Ceiling Duct Boxes.  The most likely use would be when using the Return Vent System but not connecting to the HVAC system.  It would be used to transition between the Return Vent System duct box which utilizes 4" (10cm) tubing and a 4x4 or 4x8 duct box utilizing 3" (7.6cm) vent hose.