Any circuit must have a "path" for the current flowing in it to both "go to" the receiving device (or "load") and "return from" it. In the case of an unbalanced connection; even though the return conductor may be held at ground potential by being connected to the signal ground; it still can act as a return path for the signal current.
By definition; this means that the signal ground of both the sending and receiving device must be connected together via the unbalanced connection; and this can lead to issues caused by other "interference" currents also flowing on this path. This is why balanced connections are used in audio as a means of eliminating these sources of interference.
Two sources of interference are:
- Induced noise
- Ground loops"