Typically, grounding is a must in more-complicated audio setups where pre-amps, amps, mixing gear.....are used in conjunction with each other. They need to share a common ground, but even then, ground loops can happen, causing annoying hisses and hums. In some cases, they can be removed simply by REMOVING the ground from the offending source equipment. It's normally not safe to do this (especially with microphones and stage equipment), but sometimes the show must go on.
Other times, adding a ground to an ungrounded system works. You typically don't see this sort of problem with a standard home system with 3-4 components and a main tuner/amp. Those grounds are usually just for shock protection (which is the reason for grounds in the first place). It's just that, in protecting people from shock hazards, grounds can cause odd audio problems in larger systems. Assume that the ground in home devices are just simply grounds.
