An object qualifies an action’s target, as in «قَرَأَ زَيْدٌ الْكِتَابَ». «كَانَ زَيْدٌ قَارِئًا», by contrast, locates a nominal predication in a past temporal frame. Both enrich the meaning, but they are not the same grammatical operation.
ConstructionMain effect
كَتَبَ رِسَالَةًIdentifies the object of writing
وَصَلَ لَيْلًاIdentifies the time of arrival
كَانَ زَيْدٌ مُنْطَلِقًاPlaces the attribution in the past
A careful reading separates event complements from forms that modify the entire predicative relationship.