The Sega CD manual says that the RCA jacks are for connecting the Sega CD to a stereo system or stereo TV. I guess they found that doing the sound mixing on the Sega CD made the output cleaner which would have been useful for people wanting to use it as a CD player instead of just for games. The mixing cable is necessary to use the RCA jacks on the Genesis 1 because the system does not have a sound OUTPUT on the expansion connector, only a sound INPUT. The Genesis 2 repurposed a couple of previously unused expansion pins as sound outputs, which let the model 2 Sega CD do the audio mixing without needing a patch cable.