I think they do it for customers with bulky items that you can’t comfortably scan yourself since Costco self checkouts don’t have wireless barcode scanners like Sams so the employee manning the self checkouts uses their own that basically temporarily connects to whatever terminal the customer they’re scanning for. Very helpful so I don’t have to fuddle with big packs of paper towels or soda to try to scan their barcode on the built-in barcode reader. Kind of an oversight imo Costco… you literally specialize in bulk items lol the poor worker doing self checkout scanning assistance is always running back and forth between customers