It helps even on single ended signals, when the aim is towards fair signal quality, rather than RF emissions for example.
Consider a short length of twisted pair over ground plane. The transmission line has two modes, CM and DM. Because it's used unbalanced, the modes are coupled to some extent. With one wire grounded at both ends, and the other wire the signal, that coupling is about 50%. That is, the common mode is simply (V(signal) + V(GND)) / 2.
Since the TL is short, it won't be exposed to too much interference, nor radiate too much itself; if it were long, CM would in turn couple with ambient fields, i.e. a lot of signal evaporates into space, bad news. Evidently, about half, which isn't good news even for digital signals.
Note that TL length is relative to the signal, its analog bandwidth or digital edge transition rate (not the clock rate!).
So, since CM loss depends on length and balance, if we have a short, unbalanced stretch, it's not a big deal, and we won't lose a huge amount of signal.
For example, 1dB of signal loss is merely 11% amplitude, likely no problem for a digital signal. If the loss is entirely radiated, that 11% lost is -20dB to the signal, and if the signal is some 120dBuV (i.e., about a volt), that's 100dBuV of emissions -- not a great start when your limit is, whatever, 60dBuV or so!
In simpler terms: if you lose say 1% of a signal, that's 99% still making it through, which isn't much bother to the intended signal path. And it's only 1% leaking away, who cares, right? Trouble is when the signal is large enough, relative to some emissions limit, that even that 1% causes further problems.
For a mechanical analogy perhaps -- consider the power output of a car engine. 100kW say comes out the drive shaft, and, Idunno, 200kW comes out of the exhaust pipe thermally and another 50kW is soaked up by the coolant, and so on. Well, heat isn't a bother, it's fine as long as it has somewhere to go (hence the tailpipe and the radiator). But acoustic energy is a problem. It's literally illegal (in many places) to operate a vehicle without a muffler, it's irritating for bystanders and it's damaging to the driver! It might only be a few hundred or thousand watts of actual acoustic power -- a tiny fraction of the total power, but one that, if not dealt with, becomes a big problem, for everyone!
The engine is probably perfectly fine either way, with or without a muffler; it won't make much difference to overall efficiency, or maximum power output (analogous to signal quality, as it were). The pipes themselves, might need to be tuned for best operation -- particularly true of two-cycle engines; a more direct signal quality analogy. But after that, what happens to the noise leaving the vehicle doesn't matter much to what's inside, so you can get away with various things; heh, well, I'm not sure there's an analogy for coax vs. twisted pair in terms of acoustics. But in any case, in electronics, we get concerned when the signal quality must be especially good, and when the emissions must be low.
Tim