This adapter is designed to allow a Dyson DC59 (and similar) handheld vacuum cleaner to suck up 'space saver bags' (clothing storage bags). As these vacuum cleaners have a protruding part that stops the nozzle from sealing flush with the space saver bag nozzle, you cannot get a proper seal without this piece.
Two designs are included. One is a basic nozzle with no special features, which can be used as a starting point for making other adapters and vacuum cleaner tools. The other has a limiting key (to stop the tool from rotating freely), vent holes (to allow a small amount of airflow, preventing the vacuum from pulsing as it senses low airflow), and a rotating vent cover (which allows you to adjust the amount of airflow passing through)