As for yesterday’s shot: why do we call a portrait like that “dramatic”?

Because it is dramatic. And it is that for two reasons:

• Two, you bring drama to colours and texture and contrast and skies by underexposing.
• One, the (flash-lit) subject stands out clearly against the darker background.

Consider yesterday’s sky the way it “really” was (if the camera exposed the way it wanted):

Now consider the way I shot it (2 stops “underexposed”:

See how the second shot shows detail and texture? And the dark ominous look allows me to light the subject so the subject is the “bright pixels”. Hence, a dramatic portrait.