Suppose the intention is that rain was falling at the time of Zayd's arrival. «جَاءَ زَيْدٌ وَنَزَلَ الْمَطَرُ» may report two events: he arrived and the rain began. To specify a concurrent state, «جَاءَ زَيْدٌ وَالْمَطَرُ يَنْزِلُ» is clearer.
The revision must preserve the distinction between the onset of rain and its ongoing presence at a particular moment. Rhetorical editing is not just verbal polish; it must protect the truth conditions and temporal relations of the message.