«وَمَا أَهْلَكْنَا مِنْ قَرْيَةٍ إِلَّا وَلَهَا كِتَابٌ مَعْلُومٌ»
“We never destroyed a town except that it had an appointed record.” (al-Ḥijr 15:4)
The clause “illā wa-lahā” associates each destruction with a prescribed term or decree. The circumstantial wāw helps present that condition as inseparable from the event described.
Negation and exception establish a general rule; the following circumstance specifies how that rule applies. Ignoring the conjunction and reading two detached assertions would weaken the integrated statement about the appointed order of events.