This is a specific question about a specific circuit, but I'm trying to use it as a learning experience as I'm still new to electronics and need to take practical knowledge where I can get it.
I recently purchased a box online that plugs into the cd-changer port on my stock Honda car stereo and will allow me to use any RCA stereo source as an input. It's got one plug on it and an exposed wire coming off of the box-side of the plug. The included diagram has this single hanging wire labeled as "Chassis ground when using portable device" and the step-by-step instructions say "NOTE: Chassis ground BLACK/WHITE wire ONLY when installing a battery operated audio device. (i.e. MP3 Player)"
I'm reading this as "if the AUX device you're using is not connected to the car battery, you need to ground the box," but my question is: why? Why would I have to ground the circuit box only when the AUX device is not using the same power source? Would it hurt to ground the box when the AUX is connected to the car battery? I'm not really interested in this specific device, but the principles involved in the decision.
P.S. This is the box.