وَصَلَ الْخَبَرُ، فَاطْمَأَنَّ الْقَلْبُ.
The news arrived, and the heart became reassured.
The conjunction fa marks sequence and close succession; the surrounding sense permits a cause-and-effect reading. Wāw could join two propositions without itself conveying the same temporal relation.
Assess the semantic relation between two clauses before choosing to join or separate them. This is more than a matter of punctuation or rhythm.