A 35mm film developing tank that can be loaded in daylight--outside of a darkroom or changing bag! Nifty if you don't have access to a wet darkroom.
The Rondinax was originally made by and badged as an "Agfa" product, but I'm guessing that Leitz was apparently impressed, picked up a distributorship, had it labeled as a Leitz item and distributed Rondinax tanks under the Leitz label. (Similar to the Marchioni Tiltall tripod.)