Explicitly expressing a subject can give prominence to the speaker's wonder, the gravity of an event, or sorrow over a loss. The discussion includes functions such as amazement (التعجب) and magnifying the seriousness of something (التهويل). The stated subject directs attention to what occasions the emotion.
Instead of a bare «وَقَعَ»—“[It] happened”—one might say «ذٰلِكَ الْخَطْبُ وَقَعَ»—“That momentous incident occurred,” identifying and emphasizing the event.
The effects should be distinguished. Wonder emphasizes unexpectedness, gravity emphasizes the magnitude of the situation, and regret concerns an adverse or lost outcome. Simply labelling every explicit mention “emphasis” conceals these different rhetorical intentions.