12-24 cage nuts are for equipment racks that have “Universal Rails”, sometimes referred to as square hole racks. These are the rails that have square holes punched in them, (rather than the style that has a tapped hole drilled directly in the rail). Cage nuts snap into one of these square holes creating a replaceable, threaded hole for 12-24 mounting screws.
Using cage nuts also gives you the ability to choose different thread sizes within the same installation. This is an important point when mounting a variety of rack-mount equipment from UPS devices to small network switches.