Like tape, it introduces hysteresis - as we are dealing with magnetic force and flux density. So it can pair quite nicely with tape effects as well as introducing a certain something ahead of many other plugins.
Transformers also roll off lows so they can be used to tighten up sounds.
I wouldn't call it subtle. Yes it can be as subtle as you like and unless you are metering whilst playing with it, you might be unaware just how much you can transparently clip a signal with it. But it can get into really nasty sounding distortion easily enough if you want that kind of thing. Maybe it could be perceived as subtle because the sweet spot range is so wide.
You can use it for tone, taming peaks and giving more presence and bite to sounds.
