When the appended clause reinforces a judgment that the preceding clause already states explicitly, it exemplifies confirmation of the stated meaning (taʾkīd al-manṭūq). «زَهَقَ الْبَاطِلُ» overtly declares that falsehood vanished, and «إِنَّ الْبَاطِلَ كَانَ زَهُوقًا» reinforces that demise through an account of falsehood's inherent condition.
The addition is not an unrelated fact; it makes the already expressed conclusion more secure and intelligible by presenting the same truth from an explanatory angle.