Fulll and half moon clips are required for rimless cartridges (i.e. 9mm Parabellum, .45 ACP) used in revolvers so empties can be ejected.
Half moon or fulll moon clips used with rimmed or semi-rimmed cartridges facilitate quicker loading and more positive ejection of empties, particularily in competition. There are conversions availabale for adapting cylinders for rimmed cartridges
(.38 Special, .357 Magunm, .44 Special, etc) to use full and half moon clips.(http://www.pinnacle-guns.com/revolver.asp