A nominal clause often presents its predicate with a flavor of stability or established attribution. It may not convey its temporal relation to the governing verb as directly as some verbal circumstances do. A fully formed subject–predicate clause can also sound like an independent resumption.
Circumstantial wāw can therefore reduce ambiguity by signaling that the second predication describes a state attendant on the earlier event. This is a general preference, not an unqualified ban on nominal clauses without wāw.